Coxiella burnetii (Q-Fever) Antibodies, IgG and IgM, Phase I and II with Reflex to Titer

IgG: Acute Q fever is best demonstrated by a four-fold rise in phase II IgG titers when comparing two serum samples collected 3-6 weeks apart, and testing is performed in the same laboratory at the same time. Phase I IgG titers can increase during seroconversion. However, in the case of acute infection, the phase I IgG titer should remain lower than the phase II titer. In the absence of an acute sample, a single convalescent serum sample with a phase II IgG titer greater than 1:128 in a patient who has been ill greater than 1 week, indicates probable acute Q fever.

Chronic Q fever is best demonstrated by a phase I IgG titer greater than the phase II IgG titer. Phase I and phase II IgG titers may remain elevated for months.

IgM: IgM antibodies to phase II antigens provide ancillary information to IgG titers. Phase II IgM titers develop in the same time period of phase II IgG titers and can persist for over a year.  A single phase II IgM positive result on an acute sample represents an early conversion or a false positive; testing of a convalescent serum is necessary. In the absence of an acute sample, a single convalescent serum sample with a phase II IgG titer greater than 1:128 in a patient who has been ill longer than 1 week indicates probable acute Q fever.

Chronic Q fever is best demonstrated by a phase I titer greater than the phase II IgG titer.  Phase I IgM antibodies may also develop concurrently with phase I IgG antibodies. However, in the absence of a phase I IgG titer, the diagnostic value of a phase I IgM titer is limited.  Phase I and phase II IgM and IgG titers may remain elevated for months or years after acute infection or during convalescence.