Serological Assays Assessed for Q Fever

The indirect immunofluorescence assay (IFA) is considered the reference method for diagnosing Q fever, but serology is also performed by complement fixation assay (CFA) or enzyme-linked immunosorbent assay (ELISA), however, comparability between these assays is not clear.

A total of 13 laboratories collaborated in multi-institution study to test the various methods and the results were collated by the Center for Infectious Disease Control, (RIVM; Bilthoven, The Netherlands). The Q fever cases were confirmed either by a positive polymerase chain reaction (PCR) or by seroconversion. In total, the proficiency panel consisted of 25 serum samples: five samples with an intended negative outcome, six samples from acute Q fever patients, and eight serially diluted high-positive sample from a Q fever patient.

The five Dutch laboratories that performed IFA all used the same commercially available IFA (Focus Diagnostics, Cypress, CA, USA). Six laboratories carried out a commercially available CFA (Virion-Serion; Würzburg, Germany), and five participants also performed the Virion-Serion's ELISA and one laboratory used an ELISA from Inverness Medical Innovations, (Waltham, MA, USA). Three national reference laboratories from outside the Netherlands also participated and all used an in-house IFA.

For the diagnosis of Q fever, all assays reached a specificity of 95% to 100% in the samples with an intended negative outcome. All laboratories correctly identified Q fever patients with the algorithm of the methods that were applied. The IFA, ELISA, and CFA values between laboratories using the same methods were within close range, with usually no more than two dilution differences in the reported titer. However, there were differences in sensitivity between the methods.

The IFA proved to be the most sensitive assay in detecting a past Q fever infection. All laboratories using IFA detected both immunoglobulin G (IgG) phase I antibodies (titers ranged from 1:32 to1:256) and phase II antibodies (titers ranged from1:128 to 1:512) in samples in these patients. The ELISA yielded mostly intermediate IgG phase II responses, and four of six laboratories using CFA detected only low titers of 1:10 to1:20 against phase II. No IgM response was detected with any of the applied assays.

Q fever is a highly contagious zoonotic disease caused by Coxiella burnetii, a Gram-negative obligate intracellular bacterium. Since 2007, the Netherlands has been faced with the largest outbreak of Q fever ever reported, with over 4,000 notified cases. The authors concluded that IFA, ELISA, and CFA are all suitable serodiagnostic assays for diagnosing acute Q fever. ELISAs can be an alternative for screening large sample numbers. The IFA appears to be the method of choice when high sensitivity is required, especially against phase I, and for example identifying chronic Q fever. The study was published on October 5, 2012, in the journal Diagnostic Microbiology and Infectious Disease.

Related Links:
Netherlands Center for Infectious Disease Control
Focus Diagnostics
Virion-Serion