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Zorluk: ZorDiagnostic Testing Performance

A maternal-fetal medicine network is evaluating the impact of a newly proposed sequential screening protocol for maternal autoantibodies associated with fetal heart block. The prevalence of these autoantibodies in their patient population is 2%2\%. The proposed protocol evaluates all pregnant women with a primary serological assay. Patients who test positive on the primary assay subsequently undergo a secondary advanced functional assay. Patients who test negative on the primary assay undergo no further testing.

Performance characteristics of the assays are as follows:
- Primary Assay: Sensitivity = 100%100\%, Specificity = 75%75\%
- Secondary Assay: Sensitivity = 90%90\%, Specificity = 96%96\%

If a cohort of 25,00025,000 pregnant women is screened using this protocol, how many women are expected to receive a false-positive final diagnosis?

Cevap: 245 women

Cevap

245
To find the number of final false-positive diagnoses in a sequential screening protocol, we must track the disease-free population through each testing stage. First, identify the number of true negatives in the cohort: a 2%2\% prevalence in 25,00025,000 individuals means there are 500500 true positives and 24,50024,500 true negatives. The primary assay has a specificity of 75%75\%, which corresponds to a false positive rate of 25%25\%. When the 24,50024,500 disease-free women are tested, 24,500×0.25=6,12524,500 \times 0.25 = 6,125 will incorrectly test positive. In a sequential testing algorithm, only those who test positive on the initial screen proceed to the confirmatory screen. Thus, only these 6,1256,125 false-positive women (along with the 500500 true positives) will undergo the secondary assay. The secondary assay has a specificity of 96%96\%, which corresponds to a false positive rate of 4%4\%. When the 6,1256,125 healthy women take this test, 6,125×0.04=2456,125 \times 0.04 = 245 will receive a second false-positive result. These 245245 women represent the final false-positive diagnoses for the protocol.

Adım Adım Çözüm

1
Calculate the total number of disease-free individuals in the screened cohort.
24,50024,500 women
False-positive results can only occur in individuals who do not actually have the condition. With a 2%2\% prevalence in a cohort of 25,00025,000, there are 25,000×(1−0.02)=24,50025,000 \times (1 - 0.02) = 24,500 disease-free women.
2
Determine the number of false positives generated by the primary assay.
6,1256,125 women
The primary assay has a specificity of 75%75\%, meaning it correctly identifies 75%75\% of healthy individuals. The remaining 25%25\% will receive a false-positive result (24,500×0.25=6,12524,500 \times 0.25 = 6,125).
3
Calculate the number of false positives generated by the secondary assay.
245245 women
Because this is sequential testing, only those who tested positive on the primary assay (6,1256,125 false positives) are subjected to the secondary assay. The secondary assay has a specificity of 96%96\%, meaning 4%4\% of these healthy women will incorrectly test positive again (6,125×0.04=2456,125 \times 0.04 = 245).

Anahtar Kavram

In sequential (series) diagnostic testing, the overall false positive rate is determined by applying the false positive rate (1 - specificity) of the subsequent test only to the false positives identified by the preceding test.
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Diagnostic Testing Performance

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