Chikungunya Virus or Chlamydia pneumoniae: Attributing a Post-infectious Arthropathy on a Single Immunoglobulin G in Klebsiella pneumoniae Bacteremia

A man in his 70s from a chikungunya virus-endemic region of the Caribbean presented to a community hospital with one week of low back pain and bilateral foot pain and swelling.He was diagnosed with extended-spectrum beta-lactamase-producing Klebsiella pneumoniae bacteremia from a urinary source and treated with meropenem.The foot pain and swelling, documented as present on admission, were incompletely explained by the bacteremia or by the limited unilateral arterial disease found on duplex ultrasonography; a lumbar radicular contribution was raised on the day of admission and was never formally evaluated.Arboviral serology sent on hospital day 11 returned on hospital day 15 -the first day of recurrent fevers without an identified focus -as chikungunya virus immunoglobulin G positive with immunoglobulin M negative; a polymerase chain reaction obtained outside the viremic window was not detected.Chlamydia pneumoniae immunoglobulin G was also positive.The result was entered on the problem list as an active chikungunya infection and remained there for 11 days while a fever-of-unknown-origin workup proceeded.With non-steroidal anti-inflammatory drugs contraindicated by chronic kidney disease and acetaminophen withdrawn for cholestatic liver injury, corticosteroids were started for a presumed inflammatory arthropathy, and doxycycline was given as an inpatient course.Two attributions were then documented: the hospitalist who took over care in the fourth week recorded a probable post-chikungunya arthropathy; the discharge summary attributed it to prior Chlamydia pneumoniae infection.Neither attribution is established.Both rest on a single untitered immunoglobulin G that is near-universal in this patient's demographic and cannot date an infection or link it to the presenting complaint.This case is presented as an exercise in diagnostic reasoning when serology is indeterminate.The two positive immunoglobulin G results were read three ways in one admission -the chikungunya result as an active infection on the problem list, the Chlamydia pneumoniae result as an indication for antibiotics, and finally the chikungunya result as prior infection behind a post-infectious polyarthritis -and only the last reading fits the serology.The report weighs what the panel could and could not contribute and argues that a positive immunoglobulin G in this population should be read as a framing tool rather than a diagnosis.