A large study evaluated a rapid toxin-based stool assay for diagnosing Clostridium difficile colitis. It evaluated the test against polymerase chain reaction, which served as the gold standard. The following values of the new test were obtained from a hospitalized symptomatic patient population with 50% prevalence of Clostridium difficile colitis: Which of the following is the positive likelihood ratio for the new test?
A)0)2
B)2
C)5
D)6

Question

A large study evaluated a rapid toxin-based stool assay for diagnosing Clostridium difficile colitis.  It evaluated the test against polymerase chain reaction, which served as the gold standard.  The following values of the new test were obtained from a hospitalized symptomatic patient population with 50% prevalence of Clostridium difficile colitis:  Which of the following is the positive likelihood ratio for the new test?
A)0)2 
B)2 
C)5 
D)6

Quizplus · Accepted Answer

Explanation: 11ec2b69_c0f0_7fdc_8f63_273fe144c4dd_MD0005_00 The likelihood ratio (LR) is an expression of sensitivity and specificity that can be used to assess the value of a diagnostic test. The positive likelihood ratio (LR+) represents the value of a positive test result, and the negative likelihood ratio (LR-) represents the value of a negative test result. LR+ = sensitivity / (1 - specificity). This is the probability of a patient with the disease testing positive divided by the probability of a patient without the disease testing positive. It is equivalent to: (true positive rate/false positive rate). LR- = (1 - sensitivity) / specificity. This is the probability of a patient with the disease testing negative divided by the probability of a patient without the disease testing negative. It is equivalent to: (false negative rate / true negative rate). LRs can range from 0 to infinity. A test result with a LR >1 suggests disease presence; the higher the LR, the more likely the disease presence. A test result with a LR <1 argues against the disease; the lower the LR, the less likely the disease presence. There are general guidelines for LR interpretation (Table). In this example, LR+ = sensitivity / (1 - specificity) = 0.8 / (1 - 0.9) = 8. Therefore, a positive test result would provide moderate evidence for a patient having Clostridium difficile colitis. LRs are calculated from sensitivity and specificity so they do not change with disease prevalence. Other advantages of LRs are that they can be used with tests that have more than 2 possible results; they can also be used to combine the results of multiple diagnostic tests and to calculate post-test probability for a target disorder (post-test odds = pre-test odds * LR). Educational objective: Positive likelihood ratio: LR+ = sensitivity / (1 - specificity) Negative likelihood ratio: LR- = (1 - sensitivity) / specificity Positive and negative likelihood ratios (like sensitivity and specificity) are independent of disease prevalence. References: How to use and interpret likelihood ratios (http://www.ncbi.nlm.nih.gov/pubmed/10367208) Simplifying likelihood ratios (http://www.ncbi.nlm.nih.gov/pubmed/12213147)

A Large Study Evaluated a Rapid Toxin-Based Stool Assay for Diagnosing