PPV, NPV and Troponin

If you ask most interns how to rule out NSTEMI biochemically, they will tell you to check a troponin level (plus or minus a myoglobin level) "q6 hours X3".  That 18 hours can mean the difference between admission and outpatient follow-up, and which can be a burden on patients and incur unnecessary healthcare costs.

Actual numbers obtained in the study. UA=unstable angina

An interesting paper by Scharnhorst et al. in the American Journal of Clinical Pathology suggests that myocardial necrosis can be ruled out much faster with modern troponin assays.  In a prospective, single center observational study, they evaluated the ability of a troponin of <0.06 micrograms/L or a rise in baseline troponin levels of 30% or more over two hours to diagnose myocardial infarction.  The gold standard was the synthetic diagnosis reached by the attending cardiologist.

They reported their results in terms of sensitivity, specificity, and positive and negative predictive value (PPV and NPV).  These can all be calculated from the table at right.  Sensitivity and specificity were covered in this blog recently; the positive and negative predictive values of a test are even easier to remember.  The PPV is the proportion of positive results which are accurate (in this case 30/43 = 70%) and the negative predictive value is the proportion of negative results which are accurate (in this case 94/94 = 100%).  In English, this means that in the population studied a negative test result predicted with 100% accuracy that no actual mycoardial necrosis had taken place, thus excluding the diagnosis of NSTEMI, but that a positive result was only an indicator of myocardial infarction in 70% of cases.

The important thing to note about PPV/NPV is that they depend on the prevalence of the disease.  We encountered this once previously, but, to review, this is easiest to understand if you think in extremes: 

• If the prevalence of a condition is 100%, then the characteristics of the test are irrelevant - any positive result will be accurate.  
• If the prevalence is 0%, then similarly, it doesn't matter how sensitive the test is - it's always wrong.

What this means clinically is that if you're going to make a decision based on a PPV or NPV quoted in the literature, you have to be comfortable that the prevalence in the study population was similar to the prevalence in your treatment population.

Finally, it's worth noting that these investigators also measured CK-MB and myoglobin and neither added anything to the diagnosis.  The original paper is short, free, and very much worth checking out.