### A. Parallel testing

2 screening tests are applied simultaneously:

**Disease positives:**Test positive in either of the two tests (If one test is negative, still the other test can prove positive)**Disease negatives:**Test negative in both the tests (If one test is negative, the other test must also be negative)

So, this test will have:

**Less false negatives**(increased sensitivity and increased negative predictive value or NPV)**More false positives**(decreased sensitivity and decreased positive predictive value or PPV)

### B. Series testing

After the 1st screening test is conducted, those who test positive are onyl tested with the 2nd screening test:

**Disease positives:**Those who test positive in both the tests (If 1st test is positive, he must test positive in 2nd test as well)**Disease negatives:**Those who test negative in either of the tests

So, this test will have:

**Less false positives:**Increased specificity and PPV**More false negatives:**Decreased sensitivity and NPV

#### Formulae for combined specificity and sensitivity

**A. Parallel testing:**

- Combined specificity = Specificity of test A X Specificity of test B
- Combined sensitivity = Sensitivity of test A + Sensitivity of test B – [Sensitivity of test A X Sensitivity of test B]

**B. Series testing:**

- Combined specificity = Specificity of test A + Specificity of test B – [Specificity of test A X Specificity of test B]
- Combined sensitivity = Sensitivity of test A X Sensitivity of test B

Mnemonic:Remember:

1. Sensitivity is True Positive rate

2. Specificity is True Negative rateIn probability testing, AND = multiplication and OR = Addition

`Example: Probability of getting head in coin and/or 1 in dice`

a. Head and 1 = P (head) X P (1)

b. Head or 1 = P (head) + P (1) – P (head and 1)In parallel testing:

1. Negative (A and B) = Specificity A X Specificity B

2. Positive (A or B) = Sensitivity A + Sensitivity B – [Sensitivity A X Sensitivity B]

In series testing:

1. Negative (A or B) = Specificity A + Specificity B – [Specificity A X Specificity B]

2. Positive (A and B) = Sensitivity A X Sensitivity B