# Predictive values of diagnostic tests

## What is the probability of infection after a positive test result?

In England, people reported over 260 million rapid viral tests for SARS-CoV-2. There have been almost 200 million conducted lab tests.

**Diagnostic tests are imperfect. **Sometimes, tests give a positive result to someone without an infection. Other times, they suggest a negative answer to an infected person.

**How do we calculate the tests’ ‘positive predictive value’ or ‘precision’?** That is the probability a…