This document provides a guide to COVID-19 testing. It discusses PCR testing and its sensitivity of over 85%. Sensitivity measures how often a test correctly identifies those with the disease, while positive predictive value depends on disease prevalence. Likelihood ratios are used to determine pre-test and post-test probability. Examples are given of using test results and likelihood ratios to determine the probability a patient has COVID-19. CT scans are not recommended for diagnosing COVID-19 due to their low sensitivity. Symptoms and risk factors should be used to determine pre-test probability to correlate with test results.