Editor's Note: Commentary based on Simpson AJ, Potter JM, Koerbin G, et al. Use of observed within-person variation of cardiac troponin in emergency department patients for determination of biological variation and percentage and absolute reference change values. Clin Chem 2014;60:848-54 and Aakre KM, Røraas T, Petersen PH, et al. Weekly and 90-minute biological variations in cardiac troponin T and cardiac troponin I in hemodialysis patients and healthy controls. Clin Chem 2014;838-47.

Article Summaries

Simpson et al. presented the findings of a relatively large biological variation study consisting of 283 patients who were assessed for possible acute coronary syndrome (ACS), and subsequently discharged from the emergency department (ED) with no evidence of acute cardiac disease.¹ Patients were also monitored to ensure no further events occurred in the 14 days post-hospital discharge. Each patient had a minimum of two high-sensitivity cTnI (Abbott Diagnostics) measurements, over a time period ranging from 1.5-17 hours. The repeat sampling data were used to calculate the total within-person CV (CVT), the reference change values (RCVs), the absolute RCV delta cTnI values, and the biological variation for each individual (CVi), using CVi= √ (CVT² – CVA²).

Using a 99th percentile cut point of 40 ng/L, there were similar CVi values for men (12.9%) and women (13.9%), independent of sampling times. Consistent with previous studies,^2,3,4 there was a low index of individuality (0.17), with absolute RCV values <5 ng/L for the majority (92.2%), and >10 ng/L for only 2.8% of the population. The authors reported an asymmetric RCV of -36/+56%, suggesting that clinically important changes may still occur within the reference interval, as opposed to using the absolute delta change of <10 ng/L, to safely rule out active cardiac disease up to 14 days.

These comparable results represent a validation of previous studies with smaller populations, highlighting the small index of individuality (II) seen with the cTnI assay,^2,3,4,7 while also providing important data to support the use of RCV as a complementary rule-out measurement, along with the 99th percentile, for this assay. The study is limited by key assumptions based on the lack of cardiac events at 14 days, along with the paucity of available data regarding evidence of subclinical cardiac disease, for this population.

In the study by Aakre et al., the authors examined 90-minute (total six hours) and weekly (10 weeks) biological variations in high-sensitivity troponins between 20 healthy volunteers with a mean age of 61 (range 46-68) years, and 19 chronic hemodialysis patients with a mean age of 71(35-84) years.⁵ The authors evaluated CVi, RCV, and II, using both hs-TnT (Roche Diagnostics), and hs-TnI (Abbott Diagnostics).

As expected, the hemodialysis group had increased concentrations of troponin above the 99th percentile. For the 90-minute analysis, there were decreasing concentrations for both hs-TnT groups (0.8% per hour), and by 1.7% per hour for the hs-TnI (hemodialysis patients only) group. The authors also found that, for the hs-TnI assay in clinically stable hemodialysis patients and healthy controls, the variations between CVi and CVa overlapped with the recommended diagnostic delta change values of 20% and 50%, but the overall RCV values were lower, especially for HD patients.⁵ Furthermore, for hemodialysis patients, using an absolute delta value of 7-9 ng/L for hs-TnT could potentially increase the risk of false-positive results, as the 90-minute assay variations exceed these thresholds. The relatively low II for both assays in both groups would also support the use of RCV for the evaluation of troponin amongst dialysis patients. The study was limited by variable exclusion of some measurements due to the occurrence of cardiac events, as well as unmeasurable troponin concentrations in some healthy patients.

Commentary

The diagnosis of acute myocardial infarction (AMI) is made with the presence of a rise and/or fall in cardiac troponin in a time-dependent manner, along with at least one value over the 99th percentile.⁶ The introduction of higher-sensitivity troponin assays has put focus on the need for more detailed understanding of troponin kinetics for different patient subgroups near the 99th percentiles, incorporating gender, age, and disease-specific populations. Determining the magnitude of clinically significant troponin changes in this context remains challenging, and, as demonstrated by Aakre⁵ and Simpson,¹ the newer assays also highlight the need to address key concepts surrounding biologic variation and RCVs, as well as the performance and sensitivity of absolute and relative delta (σ) changes. The findings of the recent study by Muller et al. would suggest that absolute σ change values (ranging from 6.9-9.2 ng/L) are superior to relative σ change values (of 20%), for discriminating ACS,² but the authors also specify that these findings only really apply for patients with either very low or very high troponin concentrations. For those patients with concentrations near the 99th percentile, they suggest relative σ change values and biological variability data may be more useful.

How to Interpret Biological Variation in This Context?

The RCV can be defined as the maximum difference between two consecutive results that might be caused by analytical (CVa), or biological (CVi) variation.^5,7,8 These measures are described as coefficients of variation, either occurring between values within-subjects (intra-individual, or CVi), or between subjects (inter-individual, CVG); this concept allows for quantification of the average random fluctuation around some homeostatic set point for an individual or group of individuals. The index of individuality, a measure of applicability of population-based reference ranges, can be calculated from these coefficient variables, where the II=√ (CVa²+ CVi²)/CVb. Based on these measures, we can also calculate RCVs to detect significant changes amongst patients, with a key caveat: pathologic states may alter the homeostatic set point and range for individuals with that disease and may, therefore, be inaccurate, where the RCV was derived from a healthy population.⁸

Leading from this, any RCV of greater magnitude than the σ change values used to define true ischemia, may allow for physiological variations to be incorrectly interpreted as ischemia/necrosis (false-positive). The study by Aakre et al. assessed short- (hours) and long- (weeks) term biological variation. Data was provided for the reference populations, and they found that diurnal variation in troponin detection occurred with a decline from morning to afternoon for hs-cTnT (HD and normal) and hs-cTnI (HD only). The effect on RCV was, nonetheless, only moderate. They also showed that the CVi between both divergent patient groups was relatively low for both assays, suggesting that this may also be the case for patients with baseline troponin detection within this range. With these CVi values in mind, incorporating RCV calculations using only standardized CVa values for each laboratory may offer an alternative approach to defining the allowable differences seen at these concentrations.^2,5,7

Simpson et al. confirm previous findings by Mueller et al.² that a delta hs-cTnI <10 ng/L identifies almost all patients with no active cardiac disease, according to their criteria, as well as suggesting that biological variation and CVi was less time-dependent, as did Aakre et al. By demonstrating a low index of individuality (0.17) amongst this group of patients, Simpson et al. also show that the reference intervals currently recommended may lack sensitivity to detect clinically significant troponin changes. These findings are supported by the recent study by Gore and colleagues, examining a well-defined cohort of 12,618 patients from three population-based studies: the Dallas Heart Study (DHS), the Atherosclerosis Risk in Communities (ARIC) Study, and the Cardiovascular Health Study (CHS), in which more than 10% of men aged between 65 and 74 years, had cTnT values above the current MI threshold of 14 ng/L.⁹ The authors propose that cutoff values for the hs-cTnT assay be increased to 17 ng/l for men aged 50 to 64 years and for women aged 65 years or older, and to 31 ng/l for men aged 65 years or older.⁹

Thus, these studies by Aakre and Simpson have provided important information regarding the biological variability of high-sensitivity cardiac troponins in three key populations: chronic hemodialysis, disease-free volunteers, and ED patients at risk of ACS but with no evidence of active cardiac disease, and suggest the potential utility of RCV assessment with these assays for troponin detection near the estimated 99th percentiles. For HD patients with higher baseline troponin concentrations,⁵ relative change values of 20% may lead to higher false positive rates, and RCV fluctuations may be driven more so by analytical, rather than biological, variations. Further prospective studies will be required to validate these results.

References

0.  
1. Simpson AJ, Potter JM, Koerbin G, et al. Use of observed within-person variation of cardiac troponin in emergency department patients for determination of biological variation and percentage and absolute reference change values. Clin Chem 2014;60:848-54.
2. Mueller M, Biener M, Vafaie M, et al. Absolute and relative kinetic changes of high-sensitivity cardiac troponin T in acute coronary syndrome and in patients with increased troponin in the absence of acute coronary syndrome. Clin Chem 2012;58:209-18.
3. Reichlin T, Irfan A, Twerenbold R, et al. Utility of absolute and relative changes in cardiac troponin concentrations in the early diagnosis of acute myocardial infarction. Circulation 2011;124:136-45.
4. Vasile VC, Saenger AK, Kroning JM, Jaffe AS Biological and analytical variability of a novel high-sensitivity cardiac troponin T assay. Clin Chem 2010;56:1086-90.
5. Aakre KM, Røraas T, Petersen P, et al. Weekly and 90-minute biological variations in cardiac troponin T and cardiac troponin I in hemodialysis patients and healthy controls. Clin Chem 2014; 60:838-47.
6. Kristian Thygesen; Joseph S. Alpert; Harvey D. White; on behalf of the Joint ESC/ACCF/AHA/WHF Task Force for the Redefinition of Myocardial Infarction. Universal definition of myocardial infarction. J Am Coll Cardiol 2007; 50:2173-95.
7. Khalili H, Lemos JA. What constitutes a relevant change in high-sensitivity troponin values over serial measurement? Clin Chem 2014;60:803-5.
8. Ricos C, Iglesias N, Garcia-Lario, et al. Within-subject biological variation in disease: collated data and clinical consequences. Ann Clin Biochem 2007;44:343–52.
9. Gore M, Seliger SL, deFilippi CR, et al. Age- and sex-dependent upper reference limits for the high-sensitivity cardiac troponin T assay. J Am Coll Cardiol 2014;63:1441-48.

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