Gender-specific diagnostic performance of a new high-sensitivity cardiac troponin I assay for detection of acute myocardial infarction

Abstract

Background:

The determination of cardiac troponin is essential for diagnosing myocardial infarction. A troponin I assay has recently been developed that provides the highest analytical sensitivity to date.

Methods:

The analysis included 1560 patients with chest pain, of whom 1098 were diagnosed with non-coronary chest pain, 189 with unstable angina pectoris and 273 with non-ST-segment elevation myocardial infarction. The troponin I concentration was determined on admission (0 hours) and 3 hours later. The diagnostic algorithm incorporated troponin I elevation above the gender-specific 99th percentile as well as predefined relative or absolute 3-hour changes in the troponin I concentration (delta).

Results:

The diagnostic criterion of troponin I above the 99th percentile resulted in a negative predictive value of 98.0% and 98.2% in men and women, respectively. For rule-in of non-ST-segment elevation myocardial infarction, the use of absolute deltas yielded higher positive predictive values and sensitivities compared to relative deltas. With detection rates of about 85% and 82% in men and women, respectively, non-ST-segment elevation myocardial infarction was diagnosed with a positive predictive value close to 84% in men and 80% in women.

Conclusions:

The investigational troponin I assay provides an excellent non-ST-segment elevation myocardial infarction rule out. With gender-specific differences, the application of absolute changes in troponin concentration was superior to relative changes to rule in patients with non-ST-segment elevation myocardial infarction.

Keywords

Non-ST-segment elevation myocardial infarction cardiac troponin high-sensitivity acute myocardial infarction diagnostic performance gender-specific

Introduction

The current consensus definition of myocardial infarction highlights the value of cardiac troponin determination for the diagnosis of myocardial infarction in patients presenting with chest pain. In addition to clinical evidence, electrocardiographic and imaging findings, the diagnosis of acute myocardial infarction (AMI) is defined by a 3–6-hour rise or fall of cardiac troponin I (TnI) or troponin T (TnT) with at least one value above the diagnostic cutoff, that is, the 99th percentile of a presumably healthy reference population.¹

To ensure high analytical precision, the consensus task force stipulates the use of troponin assays with a coefficient of variation (CV) of 10% or less at or below the diagnostic cutoff. Assays that meet this diagnostic standard are defined as contemporary sensitive. Assays that additionally enable troponin detection in over 50% of the general population are considered high sensitivity.² The use of contemporary sensitive and high-sensitivity troponin assays not only reduces the time interval required for diagnosing AMI but also improves diagnostic accuracy.^3–6 However, the diagnostic performance of the investigational high-sensitivity TnI assay, which provides TnI detection in up to 100% of presumably healthy individuals,⁷ has only been described in one smaller single-centre study so far.⁸ Thus, the aim of the present study was to assess the gender-specific diagnostic performance of this recently developed high-sensitivity TnI assay for diagnosing non-ST-segment elevation myocardial infarction (NSTEMI) among patients with new-onset chest pain in a real world emergency scenario.

Methods

Patients

The study assay was evaluated in a subgroup of consecutive patients enrolled at three German centres (Mainz, Koblenz and Hamburg). This study population has been described in detail before.^3,6,9 In brief, it comprised 1818 patients with new-onset chest pain admitted to the centres’ emergency rooms between January 2007 and December 2008. Clinical data assessment included blood sampling on admission (0 hours) as well as 3 and 6 hours after admission, and recording of a 12-lead electrocardiogram (ECG) at those time points. The study had been approved by the local ethics committees of Rheinland-Pfalz and Hamburg, Germany. All patients participated voluntarily and gave their written informed consent.

For the purpose of the present study, TnI concentrations were determined from stored blood samples taken at 0 and 3 hours. As this study focused on the TnI-based diagnosis of NSTEMI, patients with the ECG-based diagnosis of ST-segment elevation myocardial infarction (STEMI; n=130) were excluded. To enable quantitative assessment of serial changes in TnI, patients in whom TnI measurements at either time point were not available (n=126) and patients in whom an excessively high TnI concentration (⩾1,110,780 ng/l) was measured (n=2) were also excluded, finally resulting in a total of 1560 patients for analysis.

Assessment of the final diagnosis

The patients’ final diagnosis was independently adjudicated by two cardiologists, with a third cardiologist refereeing in discrepant cases, under consideration of all available clinical, laboratory and imaging findings, with a high number of performed coronary angiography (Table 1), inclusively. All three cardiologists were blinded to the results of the investigational TnI assay.

Table 1.

Baseline patient characteristics of the total study population.

	NCCP (n=1098)	UAP (n=189)	NSTEMI (n=273)
Age (years)	60±14	66±11	64±12
Male gender	684 (62%)	129 (68%)	206 (75%)
Risk factors
Body mass index (kg/m²)	28±5	28±4	28±5
Hypertension	773 (70%)	164 (87%)	219 (80%)
Diabetes mellitus	135 (13%)	38 (21%)	62 (23%)
Hyperlipidaemia	775 (71%)	148 (78%)	222 (81%)
Smoking status
Current smoker	242 (22%)	30 (16%)	79 (29%)
Former smoker	307 (28%)	62 (34%)	83 (31%)
Never smoker	530 (49%)	86 (48%)	103 (39%)
CAD
Parental CAD	361 (34%)	59 (34%)	88 (34%)
Known CAD	334 (31%)	112 (61%)	106 (39%)
Coronary angiography	225 (20.5%)	148 (78.3%)	265 (97.1%)
Time between chest pain onset and admission
Time of chest pain onset (hours)	4.2 (2, 11.7)	4.5 (2.1, 15.1)	4.9 (2, 19.2)
<3	425 (39%)	67 (35%)	106 (39%)
<6	652 (59%)	108 (57%)	153 (56%)
<12	826 (75%)	135 (71%)	188 (69%)
⩾12	272 (25%)	54 (29%)	85 (31%)
Biomarkers on admission
TnI (ng/l)	1.7 (1, 3.7)	2.9 (1.4, 6)	93.5 (24, 492.2)
Creatine kinase (U/l)	74 (51, 115)	73 (52, 107)	115 (78, 185)
CK-MB (ng/ml)	1 (0.6, 1.5)	1.1 (0.7, 1.7)	2.8 (1.4, 7)
C-reactive protein (mg/l)	2.3 (1.1, 5.3)	2.3 (1.3, 4.5)	3.3 (1.7, 8.7)
eGFR (ml/min for 1.73m²)	80±21	80±22	75±22
Biomarkers 3 hours after admission
TnI (ng/l)	2.2 (1.3, 5.1)	3.9 (2, 9.1)	371.7 (106.8, 1371.1)

Continuous variables are described by mean±standard deviation or median (25th percentile, 75th percentile).

TnI: troponin I concentration assayed by the investigational assay; CK-MB: MB-isoenzyme of creatine kinase; eGFR: estimated glomerular filtration rate by creatinine test; CAD: coronary artery disease; NCCP: non-coronary chest pain; UAP: unstable angina pectoris; NSTEMI: non-ST-segment elevation myocardial infarction.

AMI was defined under the terms of the prevailing consensus definition of myocardial infarction at the time of study enrollment.¹⁰ AMI was diagnosed when a rise or fall of cardiac biomarkers with at least one value above the diagnostic threshold was accompanied by clinical symptoms of ischaemia. In addition, electrocardiographic changes indicative of new ischaemia (ST-T changes or new left bundle branch block), imaging evidence of new loss of viable myocardium or detection of a culprit lesion on coronary angiography according to the Ambrose criteria¹¹ was required for the diagnosis of AMI. Cardiac troponin (TnT in Mainz and Hamburg, TnI in Koblenz) was measured with conventional in-house assays. Myocardial necrosis was defined by at least one troponin level above the concentration measurable with 10% CV (0.03 ng/ml for Roche TnT; 0.14 ng/ml for Siemens Dimension RxL TnI) in concert with an increasing or decreasing change of at least 20% within 12 hours.

Patients were diagnosed with unstable angina pectoris (UAP) if serial in-house troponin measurements were below the diagnostic cutoff and the ECG was not indicative of myocardial infarction, but myocardial ischaemia was proved by angiography-based detection of coronary artery disease or positive non-invasive stress test.

When acute coronary syndrome (STEMI, NSTEMI or UAP) was ruled out, patients were regarded as having non-coronary chest pain (NCCP). NCCP included cardiac (myocarditis, atrial fibrillation, uncontrolled hypertension, etc.) and non-cardiac causes.

Reclassification of final diagnosis

Because the primary diagnosis of AMI was based on conventional troponin assays, we assumed that some non-NSTEMI classified patients in our cohort would fulfill the criteria for NSTEMI in times of troponin assays with higher analytical sensitivity. To reduce this potential misclassification bias, we reclassified patients as having NSTEMI, who were initially diagnosed with UAP, were in need of a coronary intervention (percutaneous coronary intervention or coronary artery bypass graft surgery) and, additionally, provided the criteria for NSTEMI according to measurements with a well-established sensitive TnI assay (Siemens TnI Ultra). The NSTEMI criterion was based on the current consensus definition,¹ which is either elevation of the troponin concentration on admission and a relative change in TnI concentration of 20% or greater within 3 hours or non-elevated troponin concentration on admission, but elevated troponin 3 hours after admission in concert with a change of 50% or greater. Elevation of TnI was defined as a concentration above the gender-specific 99th percentile (47.3 ng/l for men and 32.0 ng/l for women), which was derived from the population-based Gutenberg Health Study.¹²

Biochemical analysis

The troponin assay investigated in this study was the experimental Erenna cTnI immunoassay (Singulex, Inc., Alameda, CA, USA). The assay’s lower limit of detection is 0.04 ng/l, with a 10% CV at 0.88 ng/l and a lower limit of quantification (LoQ) of 0.78 ng/l. The detection range, specified by the manufacturer, is 0.78–110 ng/l. Values below LoQ were set LoQ – 0.1 ng/l = 0.68 ng/l. All values above the upper limit have been extrapolated for quantification. All samples have been measured on 96 well plates. For each plate a standard curve (range 0–100 ng/l) was performed. Measurements with the investigational assay were performed from stored blood samples (ethylenediamine tetraacetic acid) at the facilities of Singulex, Inc., San Diego, CA, USA. All measurements were performed by experienced technical assistants blinded to patient characteristics.

Additional troponin measurements were performed with the commercially available high-sensitivity TnI assay (STAT high sensitive troponin I, ARCHITECT i2000SR; Abbott Diagnostics, USA).

As diagnostic cutoff for both assays the 99th percentile concentrations were taken from a reference population of 524 presumably healthy individuals.⁷ For the investigational TnI assay the 99th percentile concentrations were calculated with 31 ng/l (90% confidence interval 21–47 ng/l) in the overall population as well as 36 ng/l and 30 ng/l representing the 99th percentiles in men and women, respectively. For the commercially available TnI (Abbott Diagnostics) assay the 99th percentiles were 36 ng/l and 15 ng/l in men and women, respectively.

Statistics

Continuous variables are described by the mean and standard deviation or quartiles; categorical variables are presented as absolute numbers and percentages.

Troponin differences (absolute and relative deltas) were calculated as follows: for the absolute delta the absolute value of the difference in troponin concentrations at admission and 3 hours later was used. For the relative delta, the previous quantity was divided by the troponin concentration at admission and this was multiplied by 100, to obtain a percentage with respect to the first troponin measurement.

Binary diagnostic tests were produced by dichotomising troponin concentrations and by combining a delta with the troponin concentrations at admission and 3 hours later. This combination was achieved as follows: an individual was diagnosed as positive if the value of troponin at admission or 3 hours later was above a fixed cutoff and simultaneously the delta considered was above a fixed second cutoff; a person not satisfying these conditions was classified as negative.

The performance of the diagnostic algorithms considered was characterised by sensitivity and specificity, positive predictive value (PPV) and negative predictive value (NPV). Exact binomial 95% confidence intervals were computed for these quantities. Receiver operating characteristic (ROC) curves based on continuous troponin levels were computed, together with the corresponding area under the curve (AUC).

All analyses were performed using R 3.0.1 (R Core Team (2013). R: A language and environment for statistical computing; R Foundation for Statistical Computing, Vienna, Austria).

Results

Patient characteristics

The baseline characteristics of the study population are shown in Table 1. In total, the study sample comprised 1560 patients. Among these, 273 had a diagnosis of NSTEMI (18%), 189 were diagnosed with UAP (12%) and 1098 had NCCP (70%).

Diagnostic accuracy of TnI concentrations at 0 and 3 hours

The ability to discriminate between NSTEMI patients and those without NSTEMI by TnI measurement with the investigational assay on admission and after 3 hours, regardless of the time of chest pain onset, is reflected in the ROC curves shown in Figure 1. Panel (a) displays the diagnostic accuracy of TnI measurement on admission (AUC 0.94), (b) shows the diagnostic accuracy of TnI measurement after 3 hours (AUC 0.97).

Rule-out of NSTEMI by single TnI measurement

Table 2 shows diagnostic sensitivities, specificities and NPVs as well as the absolute amount of true and false positive and negative findings for troponin measurements 3 hours after admission. Application of the gender-specific 99th percentile as a diagnostic cutoff resulted in similar values for NPV (98.0% and 98.2%) and specificity (94.5% and 93.9%) in men and women. A lower diagnostic cutoff of 10 ng/l yielded higher NPVs of 99.3% and 99.0%, but reduced specificities of 85.1% and 85.9% in men and women, respectively. In contrast, when a diagnostic cutoff concentration of 50 ng/l was applied, NPVs were slightly decreased (97.3% and 97.9% for men and women, respectively), but specificities were increased (96.1% and 96.2%, respectively). The diagnostic performance for NSTEMI rule-out by TnI measurement on admission is shown in Supplementary Table 5. Overall, lower NPVs are achieved at this very early time point with 91.6% and 96.2% for men and women, respectively, when addressing the gender-specific 99th percentile. Lowering or increasing the diagnostic cutoff concentration had similar effects on NPV and specificity, as seen for the TnI measurement 3 hours after admission.

Table 2.

Rule-out of NSTEMI by TnI measurement at 3 hours after admission.

TnI (3 hours), ng/l	Men (n=1019)			Women (n=541)
	>10	>36	>50	>10	>30	>50
TP	201	190	184	63	59	57
FP	121	45	32	67	29	18
TN	692	768	781	407	445	456
FN	5	16	18	4	8	10
Sensitivity, % (95% CI)	97.6 (94.4–99.2)	92.2 (87.7–95.5)	89.3 (84.3–93.2)	94.0 (85.4–98.3)	88.1 (77.8–94.7)	85.1 (74.3–92.6)
Specificity, % (95% CI)	85.1 (82.5–87.5)	94.5 (92.7–95.9)	96.1 (94.5–97.3)	85.9 (82.4–88.9)	93.9 (91.3–95.9)	96.2 (94.1–97.7)
NPV, % (95% CI)	99.3 (98.3–99.8)	98.0 (96.7–98.8)	97.3 (95.9–98.3)	99.0 (97.5–99.7)	98.2 (96.6–99.2)	97.9 (96.1–99.0)

Gender-specific sensitivity, specificity and negative predictive values (NPVs) are given with confidence intervals (CIs) for the application of different cutoff concentrations, as well as numerous numbers of true positive (TP), false positive (FP), true negative (TN) and false negative (FN) tested patients. The applied troponin I cutoff concentration measured by the investigational assay (TnI) is the gender-specific 99th percentile (36 and 30 ng/l in men and women, respectively), as well as the arbitrary concentrations of 10 and 50 ng/l.

NSTEMI: non-ST-segment elevation myocardial infarction.

Rule-in of NSTEMI by changes in TnI concentrations

A change (delta) in troponin concentration over time is required in order to differentiate acute from chronic troponin elevation. This delta defines the diagnosis of myocardial infarction. Correspondingly, the diagnostic performance for NSTEMI rule-in was calculated for the criterion of at least one TnI value exceeding the 99th percentile, that is, 30 ng/l for women and 36 ng/l for men, on admission and/or after 3 hours in combination with a relative or an absolute delta.

Application of relative deltas

Table 3 provides diagnostic sensitivities, specificities and predictive values for the gender-specific rule-in of NSTEMI using relative deltas. Increasing the relative delta cutoff increased the PPV (i.e. the probability of a positive finding denoting a patient with NSTEMI) and decreased sensitivity (i.e. the percentage of NSTEMI patients meeting the respective criterion) for the diagnosis of AMI. The highest PPV of 87.0% was found for a delta of 200% in men, with a sensitivity of 48.5%. With respect to women, the highest PPV of 82.4% was calculated for a relative change of 400%, with a sensitivity of 20.9%. For relative deltas exceeding these values, no relevant improvement in PPVs was calculated but sensitivities were substantially decreased.

Table 3.

Rule-in of NSTEMI by relative delta changes.

	Men (n=1019; gender-specific 99th percentile: 36 ng/l)
Delta, %	⩾5	⩾10	⩾20	⩾30	⩾50	⩾100	⩾200	⩾400
Sensitivity, % (95% CI)	93.7 (89.5–96.6)	91.7 (87.1–95.1)	87.9 (82.6–92.0)	84.5 (78.8–89.1)	73.3 (66.7–79.2)	59.7 (52.7–66.5)	48.5 (41.5–55.6)	35.9 (29.4–42.9)
Specificity, % (95% CI)	94.0 (92.1–95.5)	94.1 (92.2–95.6)	95.3 (93.6–96.7)	96.1 (94.5–97.3)	96.7 (95.2–97.8)	97.7 (96.4–98.6)	98.2 (97.0–99.0)	98.3 (97.1–99.1)
PPV, % (95% CI)	79.8 (74.1–84.6)	79.7 (74.1–84.7)	82.6 (77.0–87.4)	84.5 (78.8–89.1)	84.8 (78.7–89.8)	86.6 (79.9–91.7)	87.0 (79.4–92.5)	84.1 (74.8–91.0)
	Women (n=541; gender-specific 99th percentile: 30 ng/l)
Delta, %	⩾5	⩾10	⩾20	⩾30	⩾50	⩾100	⩾200	⩾400
Sensitivity, % (95% CI)	85.1 (74.3–92.6)	82.1 (70.8–90.4)	82.1 (70.8–90.4)	80.6 (69.1–89.2)	68.7 (56.2–79.4)	49.3 (36.8–61.8)	31.3 (20.6–43.8)	20.9 (11.9–32.6)
Specificity, % (95% CI)	92.6 (89.9–94.8)	93.2 (90.6–95.3)	94.1 (91.6–96.0)	94.7 (92.3–96.6)	95.6 (93.3–97.2)	98.3 (96.7–99.3)	98.9 (97.6–99.7)	99.4 (98.2–99.9)
PPV, % (95% CI)	62.0 (51.2–71.9)	63.2 (52.2–73.3)	66.3 (55.1–76.3)	68.4 (56.9–78.4)	68.7 (56.2–79.4)	80.5 (65.1–91.2)	80.8 (60.6–93.4)	82.4 (56.6–96.2)

Sensitivity, specificity, negative (NPV) and positive predictive value (PPV) with confidence intervals (CIs) for diagnostic criterion of relative delta in combination with troponin I >99th percentile on admission and/or after 3 hours measured by the investigational assay. 99th Percentile concentration is 36 ng/l in men and 30 ng/l in women.

NSTEMI: non-ST-segment elevation myocardial infarction.

Applying a relative delta cutoff of 30% in men provided the most reasonable combination of rule-in certainty and NSTEMI detection rate with a PPV and sensitivity of 84.5%. In contrast, for a sensitivity of at least 80% in women a PPV of at most 68.4% was calculated by applying the same relative delta cutoff.

Overall, using a diagnostic algorithm with relative deltas showed gender-specific differences, with a more confident rule-in performance for men reflected by substantially higher PPVs and sensitivities for any relative delta cutoff.

Application of absolute deltas

Results of the corresponding diagnostic approach applying absolute delta cutoffs are given in Table 4. As for relative deltas, increasing the delta improved PPV and decreased sensitivity in both men and women. The use of an absolute delta cutoff value of 400 ng/l resulted in high PPVs of 91.9% (men) and 100% (women), accompanied by sensitivities of 38.3% and 35.8%, respectively. For absolute delta cutoffs of 5–100 ng/l, rule-in was more confident for men (PPVs ranging from 80.2% to 88.0%) compared to women (PPVs ranging from 63.0% to 86.5%). For absolute delta cutoffs of 200 ng/l or greater rule-in performance tended to be in favour of the female gender (89.1–91.9% vs. 92.1–100% in men and women, respectively). In men, an absolute delta of at least 20 ng/l provided the most reasonable diagnostic performance, with a PPV of 83.3% and a sensitivity of 89.3%. In women, for a sensitivity of at least 80% a PPV of 79.7% was calculated by applying an absolute delta cutoff of 30 ng/l. In comparison to relative deltas, the use of absolute deltas resulted in higher PPVs and sensitivities for any cutoff, especially in women.

Table 4.

Rule-in of NSTEMI by absolute delta changes.

	Men (n=1019; gender-specific 99th percentile: 36 ng/l)
Delta, ng/l	⩾5	⩾10	⩾20	⩾30	⩾50	⩾100	⩾200	⩾400
Sensitivity, % (95% CI)	94.2 (90.0–97.0)	93.2 (88.9–96.2)	89.3 (84.3–93.2)	85.4 (79.9–90.0)	77.7 (71.4–83.2)	67.5 (60.6–73.8)	51.5 (44.4–58.5)	38.3 (31.7–45.4)
Specificity, % (95% CI)	94.1 (92.2–95.6)	94.7 (92.9–96.1)	95.4 (93.8–96.8)	95.8 (94.2–97.1)	96.7 (95.2–97.8)	97.7 (96.4–98.6)	98.4 (97.3–99.1)	99.1 (98.2–99.7)
PPV, % (95% CI)	80.2 (74.6–85.0)	81.7 (76.2–86.4)	83.3 (77.7–87.9)	83.8 (78.1–88.5)	85.6 (79.7–90.3)	88.0 (81.9–92.6)	89.1 (82.0–94.1)	91.9 (83.9–96.7)
	Women (n=541; gender-specific 99th percentile: 30 ng/l)
Delta, ng/l	⩾5	⩾10	⩾20	⩾30	⩾50	⩾100	⩾200	⩾400
Sensitivity, % (95% CI)	86.6 (76.0–93.7)	86.6 (76.0–93.7)	86.6 (76.0–93.7)	82.1 (70.8–90.4)	76.1 (64.1–85.7)	67.2 (54.6–78.2)	52.2 (39.7–64.6)	35.8 (24.5–48.5)
Specificity, % (95% CI)	92.8 (90.1–95.0)	94.7 (92.3–96.6)	95.6 (93.3–97.2)	97.0 (95.1–98.4)	97.9 (96.2–99.0)	98.5 (97.0–99.4)	99.4 (98.2–99.9)	100.0 (98.8–100.0)
PPV, % (95% CI)	63.0 (52.3–72.9)	69.9 (58.8–79.5)	73.4 (62.3–82.7)	79.7 (68.3–88.4)	83.6 (71.9–91.8)	86.5 (74.2–94.4)	92.1 (78.6–98.3)	100.0 (79.6–100.0)

Sensitivity, specificity, negative (NPV) and positive predictive value (PPV) with confidence intervals (CIs) for diagnostic criterion of absolute delta (ng/l) in combination with troponin I >99th percentile on admission and/or after 3 hours measured by the investigational assay. 99th Percentile concentration is 36 ng/l in men and 30 ng/l in women.

NSTEMI: non-ST-segment elevation myocardial infarction.

Discussion

We assessed the diagnostic performance of a new high-sensitivity TnI assay to diagnose NSTEMI among patients with new-onset chest pain. Rule-out of NSTEMI was achieved by a single TnI measurement 3 hours after admission. Irrespective of gender, this approach yielded a rule-out diagnostic certainty (NPV) of 98% or greater, with a non-NSTEMI detection rate (specificity) of about 94% when the gender-specific 99th percentile was used as diagnostic cutoff. Arbitrarily lowering the cutoff concentration to 10 ng/l entailed an improved NPV of 99% or greater in both gender at the cost of a markedly reduced specificity. For rule-in of NSTEMI, the use of absolute deltas yielded higher certainties (PPVs) and detection rates (sensitivities) compared to relative deltas, but this improvement was more distinctive in women. In men a diagnostic algorithm with either an absolute (⩾20 ng/l) or a relative delta (⩾30%) enabled the rule-in of NSTEMI with 83–85% certainty (PPV) in 85–89% NSTEMI patients (sensitivity). The optimal rule-in performance in women was calculated for an absolute delta of 30 ng/l or greater with a PPV of about 80% and a sensitivity of 82%.

The investigational TnI assay

The improvement in analytical precision of troponin detection ushered in the era of sensitive troponin assays. Among those, the TnI assay used in the current study represents probably the latest developmental stage with the capability to quantify TnI concentrations in up to 100% of individuals in the general population.⁷ To date, the assay’s characteristics have been evaluated in such presumably healthy reference populations^7,13–15 as well as a prognostic biomarker after NSTEMI.¹⁶ The clinical benefit for patients with acute chest pain remains unclear and has only been investigated by one single-centre study of including 381 subjects.⁸ The current study is focused on the assay’s performance with a clinically relevant algorithm for diagnosing NSTEMI in a real world emergency room patient cohort. We have demonstrated that the investigational assay provides adequate discrimination between NSTEMI and non-NSTEMI with high diagnostic accuracy.

Definition of cutoff concentrations

In the literature, the 99th percentile concentration for the investigational TnI assay ranges from 7 to 40 ng/l.^7,13 The gender-specific 99th percentile value used in the present study was derived from the work of Apple and colleagues.⁷ In that study the 99th percentile value was determined from a well-defined reference population. Still, given such discrepancies in 99th percentile values for the same assay, the reliability of the 99th percentile as diagnostic cutoff remains doubtful. Several studies could demonstrate that an exclusion of presumably ‘not healthy’ participants from the reference population leads to lower 99th percentile troponin values.^12,17 To avoid this ambiguity, we could show in our study that the application of alternative cutoff values is also feasible for diagnostic purposes. Application of an arbitrarily chosen lower TnI concentration of 10 ng/l entailed a lower number of false negative findings (reflected by higher NPV) at the cost of a higher number of false positive findings (reflected by lower specificity). Further studies will have to show if these alternative cutoff values can be validated in other patient populations with chest pain.

Reclassification of final diagnosis

The primary definition of myocardial infarction in our cohort was based on troponin assays with lower analytical sensitivity compared to the troponin assay under investigation. Therefore, patients who would be classified as NSTEMI by the application of a troponin assay with higher sensitivity could have been misclassified as UAP or NCCP. However, none of the individuals who were classified with NCCP was in need of a coronary intervention, although the rate of coronary angiography in these patients was high (20%). This indicates the high accuracy of the NCCP diagnosis in our cohort. Thus, for the current analysis we did not adjust the final diagnosis in individuals who were diagnosed with NCCP. In contrast, we reclassified individuals as having NSTEMI, who were initially diagnosed with UAP, were in need of coronary intervention and additionally provided elevated TnI assayed by a more sensitive assay. According to this additional criterion, a total of 26 patients with the primary diagnosis of UAP were reclassified as NSTEMI. These patients represent additional individuals suffering from myocardial infarction with a need for urgent invasive treatment.

Clinical implications

In clinical routine, there are two fundamental requirements for a troponin assay. First, the rule-out of patients without NSTEMI with high diagnostic certainty and, second, a precise rule-in of, ideally, all patients suffering from NSTEMI.

Accordingly, adequate rule-out of NSTEMI requires the number of false-negative findings to be as low as possible reflected by high NPV. In addition, rule-out should come along with high specificity, meaning the number of false positive results ought to be as low as possible. By using the simple criterion of the 3-hour TnI measurement exceeding a certain cutoff concentration, the investigational assay provided an NPV of 98% or greater with a specificity of about 94% for both genders. According to the literature, for other high-sensitivity troponin assays slightly higher NPVs of 99–100% but lower specificities of 76.7–90.4% are described in different cohorts.^4–6,18 The diagnostic performance of the commercially available high-sensitivity TnI (Abbott Diagnostics) assay in our study cohort supports this finding of a minor inferiority of the investigational assay to rule-out patients without NSTEMI, especially at very early time points (on admission) (see Table 2 and Supplementary Tables 5 and 6).

The confidence that a patient is suffering from NSTEMI is reflected by PPV, which is the probability of a positive result indicating a patient with NSTEMI. As our data show, increasing the diagnostic delta threshold improves PPV. This gain of certainty comes at the cost of a decreased sensitivity. We demonstrated in our study that the application of an absolute instead of a relative delta improves both PPV and sensitivity. Maximum PPVs of 91.9% and 100% for men and women, respectively, were calculated for very high absolute deltas (⩾400 ng/l), which was the case in only 36–38% of patients with NSTEMI. The ideal absolute delta cutoff for rule-in of NSTEMI was 20 ng/l in men (PPV 83.3%, sensitivity 89.3%) and 30 ng/l in women (PPV 79.7%, sensitivity 82.1%). It has also been shown for high-sensitivity assays that for the diagnosis of AMI absolute deltas are superior compared to relative deltas.^19–21 This superiority of absolute deltas is most likely based on its independence from the baseline troponin concentration. In contrast, the use of relative deltas may lead to false negative findings in patients with high troponin baseline concentrations and, due to an overlap with the analytical and biological troponin variation, to false positive findings in patients with low troponin baseline concentrations. In studies that apply absolute delta changes in troponin assayed by contemporary sensitive and high-sensitivity assays (Roche hs-TnT and Siemens TnI Ultra), PPVs of 49–64% were calculated for a NSTEMI-detection rate (sensitivity) of 90%.^20–22 The amount of absolute delta used in these studies ranged from 7.0 to 9.2 ng/l (TnT) and 20 to 55 ng/l (TnI). In our study, for a diagnostic sensitivity in the same range (89.3–93.2%), the investigational TnI assay yielded a higher PPV of 83.3–81.7% in men by using an absolute delta of 20 ng/l or greater and 10 ng/l or greater, respectively. Women provided a slightly inferior rule-in performance with a PPV up to 73.4% for a maximum sensitivity of 86.6% by an absolute delta change of 20 ng/l or greater. We also performed a direct comparison in the same population with the high-sensitivity TnI (Abbott Diagnostics) assay. This assay yields for a detection rate of 90.3% a similar PPV of 84.3% by applying an absolute delta change of 20 ng/l or greater in men. Certainly, in women the PPV seemed to be noticeably lower compared to the investigational assay with 53.1–64.1% (sensitivities 86.4–84.7%) by applying an absolute delta change of 10 ng/l or greater and 20 ng/l or greater, respectively. For higher changes in troponin concentration these potential differences appear to become less pronounced (see Table 4 and Supplementary Table 7). However, to compare the diagnostic value and predictive impact of different troponin assays and/or diagnostic algorithms adequately a prospective analysis under identical baseline conditions should be performed.

Gender specificity

According to the third universal definition of myocardial infarction, the use of gender-specific 99th percentile values are recommended for diagnosing AMI in order to balance gender-specific differences.¹ Whereas rule-out had no relevant sex-specific discrepancies, rule-in performance differed between men and women in the current study. The application of low absolute deltas (⩾5–100 ng/l) resulted in higher PPVs in men, whereas PPVs for high absolute deltas (⩾200 ng/l) tended to be in favour of the female gender. For almost any (absolute or relative) delta cutoff the diagnostic sensitivity was higher in men, indicating that fewer women with myocardial infarction can be detected even by the most sensitive assayed troponin. The impact of these gender-specific diagnostic differences on clinical outcome needs to be investigated in further studies.

Limitations

There are limitations inherent in the present study. First, differences in the time from chest pain onset were not incorporated into our analyses. Second, the study population consists of white Europeans only; therefore, our results may not be transferable to other ethnicities. Basically, evaluation of troponin assays in a cohort with a troponin-based diagnosis of NSTEMI might lead to a diagnostic distortion. To minimise this bias, the final diagnoses were additionally based on clinical and imaging findings as well as on a high number of coronary angiographies with or without the need for intervention.

Conclusions

This is the first investigation of this new high-sensitivity TnI assay for the gender-specific detection of NSTEMI in a real world chest pain unit scenario. The assay enables an adequate way to rule out patients with NSTEMI. Moreover, the gender-specific application of absolute deltas supports the diagnosis of NSTEMI.

Footnotes

Acknowledgements

NS and FJB contributed equally to this manuscript. All authors take responsibility for all aspects of the reliability and freedom from bias of the data presented and their discussed interpretation.

Conflict of interest

SB has received research funding from Abbott, Abbott Diagnostics, Bayer, Boehringer Ingelheim, SIEMENS and Thermo Fisher, formerly BRAHMS. He received honoraria for lectures from Abbott, Abbott Diagnostics, Astra Zeneca, Bayer, Boehringer Ingelheim, Medtronic, Pfizer, Roche, SIEMENS Diagnostics, SIEMENS, Thermo Fisher and as a member of advisory boards and for consulting for Boehringer Ingelheim, Bayer, Norvartis, Roche and Thermo Fisher/BRAHMS. TK has received honoraria from Abbott and is consulting for Roche and Thermo Fischer. All other authors have no conflict of interest.

Funding

The present study was supported by the ‘Schwerpunkt Vaskuläre Prävention’ of the Johannes Gutenberg-University of Mainz.

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