Sensitivity&; specificity tests

1. Sensitivity
•Proportion of patients with disease who are tested positive with a test
•A 100% sensitive test will not have any false negative results (although it may have a high rate of false positive results)
•Therefore, a negative result of a highly sensitive test means it is likely to be a true negative (it rules out the disease)
“SN-OUT”
Specificity
•Proportion of patients without disease who are tested negative with a test
•A 100% specific test will not have false positive results (although it may have high rate of false negative results)
•Therefore, a positive result of a highly specific test means it is likely to be true positive (it rules in the disease)
“SP-IN”
Sensitivity Disease+ve Disease-ve
Test +ve a (TP) b(FP)
Test–ve c (FN) d (TN)

2. TP = True positive
FN = False negative
FP = False positive
TN = True negative
Sensitivity = (a)/(a+c)
Positive Predictive Value Disease+ve Disease-ve
Test +ve a (TP) b(FP)
Test–ve c (FN) d (TN)
TP = True positive
FN = False negative
FP = False positive
TN = True negative
Positive PV = (a)/(a+b)
Specificity Disease+ve Disease-ve
Test +ve a (TP) b(FP)
Test–ve c (FN) d (TN)
TP = True positive
FN = False negative
FP = False positive
TN = True negative
Specificity = (d)/(b+d)

3. Negative Predictive Value Disease+ve Disease-ve
Test +ve a (TP) b(FP)
Test–ve c (FN) d (TN)
TP = True positive
FN = False negative
FP = False positive
TN = True negative
Negative PV = (d)/(c+d)
Example: Troponin assays
•First generation assay: cut-off 0.5 microgm/l
•3rdgeneration assay: 0.05 –0.10 microgm/l
•High-sensitive troponin (hsTn): 0.0030 microgm/l
•High-sensitive Roche Elecsys: 0.0014 microgm/l
•The diagnostic sensitivity of hsTnassays (ability to rule-out MI) are of the order of 90–95% when tested at the point of admission (still misses 5 -10% of cases)
Ref: Gamble et al, Br J Cardiol. 2013;20(4)
Causes of elevated troponins
•Myocardial ischemic conditions
•ACS
•Myocardial ischemic conditions other than ACS
•Systemic conditions
•Myocardial injury without ischemic insults
•Systemic conditions –renal failure, sepsis
•Specific identifiable precipitants –cardiac contusion, burns >30% BSA

4. Sensitivity and Specificity
•A trade-off
•When sensitivity increases, specificity decreases
•When specificity increases, sensitivity decreases