Thursday, November 6, 2014

False Positive

A 60 year old man who is several weeks out from a bone marrow transplant for multiple myeloma develops fever, shortness of breath, tachycardia, and hypotension. He is started on vancomycin, levofloxacin, and piperacillin-tazobactam and then admitted to the intensive care unit. His lactate is elevated, and he is fluid resuscitated. A flu test is negative. A diagnostic test is sent to look for a specific polysaccharide in the serum that would change the antibiotic management. It comes back positive, but your infectious disease consultant says, "Wait, that test could be a false positive."

Challenge: What is the test and why is it unreliable here?

Image of bone marrow transplant shown under Creative Commons Attribution Share-Alike License, from Wikipedia.

2 comments:

Anonymous said...

galactomannan; cross reaction w/ Zosyn (some batches)

Craig Chen said...

good one!
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False Positive

Patients who have had bone marrow transplants are at risk for aspergillus infection. Galactomannan antigen detection is a serum test that detects Aspergillus cell wall polysaccharides. However, false positives can be seen in patients receiving beta lactam antibiotics such as piperacillin-tazobactam.

Source: UpToDate.