This paper outlines a practical approach to assist physiotherapists to interpret the results of diagnostic or screening tests. Diagnostic tests are used during clinical assessment to increase or decrease the clinician's estimate of the likelihood that a client has a particular condition. A negative result for a test that is 100% sensitive can rule a condition out (SnOUT), and a positive result for a test that is 100% specific can rule a condition in (SpIN). However, tests are rarely 100% accurate, and false-positive and false-negative results can occur. The examining therapist needs to estimate the probability that a client has a particular condition (the pre-test probability), then estimate the extent to which they are more or less certain given a positive or negative test result (the post-test probability). The likelihood ratio, which combines the information provided by a test's sensitivity and specificity, is the most useful tool for the clinical interpretation of test results.