An Army Sergeant With Mysterious Pain: Crack the Case

Christopher L. Tracy, MD


November 10, 2014

Ordering Screening and Diagnostic Tests

Appropriate use of screening and diagnostic tests is an important component of providing high-value healthcare. Two general principles have been postulated for the appropriate use of diagnostic tests.[6]

The first is that you should not perform diagnostic tests if the results would not change management. By way of an example used in a previous publication,[7] say you obtained a chest radiograph 4 weeks after a diagnosis of pneumonia in a patient who has responded clinically to treatment.Results of the chest imaging will not affect management because radiographic abnormalities may take up to 6-8 weeks to resolve.So, the test incurs costs with little or no benefit to the patient.

The second general principle is that when the pretest probability of a disease is low, a false-positive test result is more likely than a true positive. Using Lyme serology testing as an example, patients presenting solely with nonspecific symptoms, such as chronic fatigue or chronic diffuse arthralgias and myalgias, should not be tested for Lyme disease even if they reside in endemic areas because the probability of disease is low.[8] Rather, you should test only in patients living in or who have been exposed to endemic areas when they have risk factors for tick exposure or have definite tick exposure, and have symptoms consistent with Lyme disease.

The musculoskeletal manifestations of Lyme disease include brief attacks of arthralgias or intermittent or persistent episodes of arthritis in one or a few large joints at a time, especially the knee. Lyme testing in the absence of these clinical features is more likely to turn up false-positive results and may lead to unnecessary follow-up and therapy.[9] Studies have shown that in endemic areas, appropriate testing for Lyme disease occurs in only 20%-40% of potential cases, whereas in nonendemic areas, testing occurs in only 10% of the cases.[9] In our patient, the lack of objective clinical examination findings and lack of exposure history would support a low pretest probability of disease.

On further review of past laboratory results, you notice that this patient had previously tested positive for ANA at a 1:80 titer.


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