Source:

Chapter adapted and updated, with permission, from Nicoll D et al. Guide to Diagnostic Tests, 7th ed. McGraw-Hill, 2017.

A practical way to calculate the posttest probability of disease is to use the LRs and odds-probability approach.

LRs combine both test sensitivity and specificity into a single measure (a mathematical description of the strength of a diagnostic test), which helps evaluate and interpret a diagnostic test. LRs indicate how many times more (or less) a test result is to be found in persons with disease compared with persons without disease. There are two types of LR—LR positive and LR negative, calculated by the following formulas: When test results are dichotomized using a single cutoff value to divide "positive" from "negative," every test has two LRs, one corresponding to a positive test (LR+) and one corresponding to a negative test (LR–): For continuous measures, multiple interval LRs (iLRs) can be defined to correspond to ranges or intervals of test results. The iLR for a test result interval is the probability of a result in that same interval for a disease-positive patient divided by the probability of a result in the same interval for a disease-negative patient. Given the pretest probability of disease and the test result, the iLR is used to calculate the posttest probability (Table e2–6).

Table e2–6. Interval LRs for serum ferritin as a test for iron deficiency anemia.
Serum Ferritin (mcg/L) Interval LR for Iron Deficiency Anemia
≥ 100 0.08
45–99 0.54
35–44 1.83
25–34 2.54
15–24 8.83
≤ 15 51.85

Data modified from Guyatt G et al. Laboratory diagnosis of iron deficiency anemia. J Gen Intern Med. 1992 Mar–Apr;7(2):145–53.

LRs can be calculated using the above formulas. They can also be found in some textbooks, journal articles, and online programs (such as www.thennt.com) (see Table e2–7 for sample values). LRs provide an estimation of whether there will be significant change in pretest to posttest probability of a disease given the test result, and thus can be used to make quick estimates of the usefulness of contemplated diagnostic tests in particular situations. An LR of 1 implies that there will be no difference between pretest and posttest probabilities. LRs of more than 10 or less than 0.1 indicate large, often clinically significant differences. LRs between 1 and 2 and between 0.5 and 1 indicate small differences (rarely clinically significant).