USMLE Step 1 Question 296

Question:

A 35 year old woman presents with recent onset constitutional symptoms, including daily fevers, a 5 kg weight loss, and fatigue. She also has been “coughing up blood” and has had occasional sinus headaches and runny nose. An initial chest X-ray shows cavitations. A PPD is read as negative. You suspect the patient may have Wegener’s granulomatosis, and want to test her cANCA level. You tell her that in general the probability a positive test result means that she actually has the disease are most dependent on

Correct Answer: C.  The prevalence of Wegener’s in people with her symptoms

The probability that a patient is truly positive for a disease given a positive test result is known as the positive predictive value (PPV). The PPV changes with the prevalence of the disease in a population. When the disease is very prevalent, the PPV will increase; when it is very uncommon, it will decrease. The opposite is true of the negative predictive value.

Sensitivity and specificity of a test are fixed test characteristics that are not affected by disease prevalence.

The other choices are not relevant for this disease and do not answer the question.