One-Test Strategy
|
Two-Test Strategy
|
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TP1=
|
N x E x SenA
|
TP2 =
|
TP1 x SenB
|
TN1 =
|
(N x (1 – E)) – (N x (1 – E) x (1 – SpecA))
|
TN2 =
|
TN1 + (FP1 x SpecB)
|
FP1 =
|
N x (1 – E) x (1 – SpecA)
|
FP2 =
|
FP1 x (1 – SpecB)
|
FN1 =
|
N x E x (1 – SenA)
|
FN2 =
|
FN1 + (TP1 x (1 – SenB))
|
PPV1 =
|
TP1 / (TP1 + FP1)
|
PPV2 =
|
TP2 / (TP2 + FP2)
|
NPV1 =
|
TN1 / (TN1 + FN1)
|
NPV2 =
|
TN2 / (TN2 + FN2)
|
POR1 =
|
TP1 / FP1
|
POR2 =
|
TP2 / FP2
|
-
N Population size, E Prevalence of infection, TP True positive, SenA Assay A sensitivity, SpecA Assay A specificity, TN True negatives, SenB Assay B sensitivity, SpecB Assay B specificity, FP False positives, PPV Positive predictive value, FN False negatives, NPV Negative predictive value, POR Ratio of true to false positive tests