Ch04-9781975113285

Useful Formulae   1 5

MEDICAL EPIDEMIOLOGY

• Yeah, can you believe this stuff actually matters? • The letters in the following refer to a standard 2 × 2 table presented in Figure 4-1. • Sensitivity: the percentage of patients with the target disease/ condition who have a positive result [A/(A + C)]. The greater the sensitivity, the more likely the test will detect patients with the disease. High sensitivity tests are useful clinically to rule OUT a disease (SnOUT) (i.e., a negative test result would virtually exclude the possibility of the disease). • Specificity: the percentage of patients without the target disease/ condition who have a negative test result [D/(B + D)]. Very spe- cific tests are used to confirm or rule IN the presence of disease (SpIN). • Positive predictive value : the percentage of persons with positive test results who actually have the disease/condition [A/(A + B)]. • Negative predictive value : the percentage of persons with negative test results in which the disease/condition is absent [D/(C + D)]. • Number needed to treat : the number of patients who need to be treated to achieve one additional favorable outcome; calculated as 1/absolute risk reduction, rounded up to the nearest whole number • Number needed to harm : the number of patients who, if they received the experimental treatment, would lead to one additional person being harmed compared with patients who receive the con- trol treatment; calculated as 1/absolute risk increase