A 26-year-old woman comes to the office due to several weeks of fatigue, headaches, joint pain and swelling, light sensitivity, and occasional periods of low-grade fever. The patient has no other medical issues. She works as a systems engineer for a major airline. Family history is unremarkable. The diagnosis of systemic lupus erythematosus (SLE) is considered, and an antinuclear antibody (ANA) assay is obtained. The prevalence of SLE in patients from whom an ANA test is obtained is 3.2%. The sensitivity of ANA for SLE is 96%, and the specificity is 91%. Assuming that this patient has SLE, how likely is she to have a positive ANA result compared with a patient without SLE having a positive ANA result?

Question

A 26-year-old woman comes to the office due to several weeks of fatigue, headaches, joint pain and swelling, light sensitivity, and occasional periods of low-grade fever.  The patient has no other medical issues.  She works as a systems engineer for a major airline.  Family history is unremarkable.  The diagnosis of systemic lupus erythematosus (SLE) is considered, and an antinuclear antibody (ANA) assay is obtained.  The prevalence of SLE in patients from whom an ANA test is obtained is 3.2%.  The sensitivity of ANA for SLE is 96%, and the specificity is 91%.  Assuming that this patient has SLE, how likely is she to have a positive ANA result compared with a patient without SLE having a positive ANA result?

Quizplus · Accepted Answer

Explanation: 11ec2b69_c0f0_cdfd_8f63_afe61b34b878_MD0005_00 The likelihood ratio (LR) is the probability of finding an event (eg, positive or negative results on a diagnostic test) in patients with the disease divided by the probability of the same finding in patients without the disease. LRs are a different method of assessing the value of a positive or negative diagnostic test in the clinical setting. The positive LR (LR+) is the probability of a patient with the disease testing positive divided by the probability of a patient without the disease testing positive. Conversely, the negative LR (LR−) is the probability of a patient with the disease testing negative divided by the probability of a patient without the disease testing negative. Contrary to the positive predictive value and the negative predictive value, LRs are independent of the prevalence of disease. Therefore, LRs can be calculated from sensitivity and specificity estimates: LR+ = sensitivity / (1 − specificity) LR− = (1 − sensitivity) / specificity LRs are useful for determining how a positive or negative diagnostic test result influences a patient's likelihood of having the disease. Consequently, the LR+ can answer the question, "If a patient has condition X (eg, systemic lupus erythematosus [SLE]), how likely is the patient to have a positive test result (eg, positive antinuclear antibody [ANA] result) compared with a patient without the condition having a positive test result?" In this case, the sensitivity and specificity of ANA for SLE are 96% and 91%, respectively. Based on these values, the LR+ for this patient is: LR+ = 0.96 / (1 − 0.91) = 10.67 Therefore, assuming this patient has SLE, she is 10.7 times more likely to have a positive test result compared with a patient without SLE. The following general guidelines are considered when interpreting LRs: general guidelines LR <1 indicates a decreased likelihood that the disease is present LR = 1 indicates no change in the likelihood of disease (Choice C) LR >1 indicates an increased likelihood that the disease is present (Choice A) This value represents the LR−, which answers the question, "If the patient has SLE, how likely is she to have a negative ANA result compared with a patient without SLE having a negative ANA result?" LR− = (1 − 0.96) / 0.91 = 0.04 Educational objective: The likelihood ratio (LR) is the probability of a patient with the disease having a particular finding (eg, positive or negative test) divided by the probability of a patient without the disease having the same finding. The formula for a positive LR is sensitivity / (1 − specificity); the formula for a negative LR is (1 − sensitivity) / specificity. Positive and negative LRs can be calculated from sensitivity and specificity values or a contingency table.

A 26-Year-Old Woman Comes to the Office Due to Several