Accuracy and Precision

Sensitivity and specificity of lab tests vary by the accuracy and precision of the test. The table below demonstrates how sensitivity and specificity varies by proportion of atypical lymphocytes in a patient with clinically suspected infectious mononucleosis.

Sensitivity and Specificity: the differential in patients with symptoms of mononucleosis and a positive heterophile antibody (Ebell)
Patient with clinical signs of mono and positive heterophile antibody	Sensitivity %	Specificity %
≥ 10% atypical lymphocytes	75	92
≥ 20% atypical lymphocytes	56	98
≥ 40% atypical lymphocytes	25	100

The high specificity of having ≥ 40% atypical lymphocytes means that you could, with a high degree of accuracy, tell a patient with this result that he/she has infectious mononucleosis. Notice that as a test’s specificity increases, its sensitivity decreases. What does the decrease in sensitivity mean? A high specificity of a test means that if a person tests positive, he/she will actually have the disease. However no test is perfect, and a lower sensitivity means you will miss some people who have the disease, but have a negative test result. Let us look at another example. Hemoglobin is commonly used as a screening test for iron deficiency. Iron deficiency is common in certain age groups - the toddler and adolescent—because of rapid growth and iron poor diets, and in menstruating females of all ages because of increased physiologic losses. Hemoglobin is the test used most often because it is inexpensive and widely available, but low hemoglobin it is not very sensitive or very specific for iron deficiency. In a cohort study of 1,289 children 12-35 months of age, investigators evaluated CBC, ferritin, transferrin saturation, and free erythrocyte protoporphyrin (EPP) level to look for iron deficiency and anemia. In this sample, the prevalence of all anemias was 8%, the prevalence of iron deficiency was 9%, and the prevalence of iron deficiency anemia was 3%. When only the measure of Hb is used, many actual iron deficient children would be missed and many anemic children would be treated unnecessarily with iron. The sensitivity of a hemoglobin level in toddlers in detecting iron deficiency is 30% (White, 2005).

The mono spot or heterophile antibody test is very sensitive for diagnosing mono in people over 10 years of age but has a sensitivity of less than 50% in children less than 2 years. The range of sensitivity for this test is between 71% to 90% (Linderholm, Boman, Juto, Linde, 1994). However, the sensitivity of heterophile antibody testing is lower in children under 12, ranging from 25-50% (Bruu et al, 2000). During the first week, there is 25% false negative rate (Ebell). Based on this information answer the following two questions.