1. What is Number Needed to Test (NNT)?

Definition: NNT is the number of patients who need to be tested to prevent one additional bad outcome.

Purpose: It helps healthcare professionals evaluate the effectiveness of diagnostic tests and screening programs.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( NNT \) — Number needed to test (dimensionless)
• \( ARR \) — Absolute risk reduction (dimensionless, between 0 and 1)

Explanation: The reciprocal of the absolute risk reduction gives the number of patients that need to be tested to prevent one adverse outcome.

3. Importance of NNT Calculation

Details: NNT helps determine the clinical utility and cost-effectiveness of diagnostic tests by quantifying how many people need to be tested to prevent one bad outcome.

4. Using the Calculator

Tips: Enter the absolute risk reduction (ARR) as a decimal between 0 and 1 (e.g., 0.05 for 5%). The value must be > 0 and ≤ 1.

5. Frequently Asked Questions (FAQ)

Q1: What does a lower NNT indicate?
A: A lower NNT means the test is more effective, as fewer people need to be tested to prevent one bad outcome.

Q2: How is ARR calculated?
A: ARR = Control event rate - Experimental event rate (difference in risk between groups).

Q3: What's a good NNT value?
A: Lower is better. NNT of 2-5 is excellent, 5-15 is good, while >25 may be less clinically useful.

Q4: Can NNT be less than 1?
A: No, since ARR cannot exceed 1, NNT cannot be less than 1.

Q5: How does NNT differ from NNT (Number Needed to Treat)?
A: NNT refers to testing/diagnosis, while NNT refers to treatment. Both measure effectiveness but in different contexts.