Serial and parallel testing
- Concept
Combining two tests changes performance: serial testing raises specificity, parallel testing raises sensitivity.
What it is
When two tests are used, the rule for combining their results sets the performance. In parallel (both are done and one positive is enough), fewer diseased people are missed: sensitivity rises and specificity falls. In series (the second is done only if the first is positive, and both must be positive), fewer healthy people are flagged: specificity rises and sensitivity falls. Two-step screening is a classic use of serial testing.
Worked example
- Data: test A with 80% sensitivity and 90% specificity; test B with 70% and 95%. They are assumed independent.
- Parallel sensitivity = 1 − (0.20 × 0.30) = 1 − 0.06 = 0.94 (94%).
- Parallel specificity = 0.90 × 0.95 = 0.855 (85.5%).
- Serial sensitivity = 0.80 × 0.70 = 0.56 (56%).
- Serial specificity = 1 − (0.10 × 0.05) = 1 − 0.005 = 0.995 (99.5%).
- Reading: in parallel few diseased people are missed; in series there are almost no false positives, but almost half the cases are lost.
The combinations in the example
| Strategy | Rule for a positive | Sensitivity | Specificity | Use |
|---|---|---|---|---|
| Test A alone | A positive | 80% | 90% | Baseline |
| Parallel | A or B positive | 94% | 85.5% | Emergency care, not missing cases |
| Serial | A and B positive | 56% | 99.5% | Confirmation, two-step screening |
Markers
- serial
- parallel
In practice
- In emergency care several tests are ordered at once so as not to miss a serious diagnosis (parallel logic).
- In infection screening, an initial positive is confirmed with a second, different test before the diagnosis is reported (serial logic).
Common pitfalls
- Applying the formulas to tests that measure the same thing and are not independent: the real gain is smaller.
- Mixing up which strategy raises sensitivity and which raises specificity.
- Forgetting that serial testing saves tests because only positives get the second one.
Test yourself
Think of the answer, then tap the question to see it.
Which strategy raises overall sensitivity?
Parallel testing: anyone positive on either test counts as positive.
Two independent tests, each with 80% specificity, are used in series. What is the combined specificity?
1 − (0.20 × 0.20) = 1 − 0.04 = 0.96, that is 96%.
Why is two-step screening a serial strategy?
Because only people positive on the first test go on to the second, and the diagnosis needs both to be positive.
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Sensitivity, Specificity, and Predictive Values: Foundations, Pliabilities, and Pitfalls in Research and Practice (Frontiers in Public Health, 2017); Measures of Diagnostic Accuracy: Basic Definitions (EJIFCC, 2009).
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Representative model, for educational purposes only.