Diagnosing reactive arthritis - Role of clinical setting in the value of serologic and microbiologic assays

Some important conclusions and considerations can be made from the data presented. First, none of the tests is useful if the clinical picture is not suggestive of ReA and if other diagnoses have not been excluded; in this case, the low pretest probability necessarily results in a low posttest probability. This means that tests used for the identification of triggering bacteria are only useful if applied by doctors familiar with the clinical picture of ReA and with the differential diagnosis of rheumatic diseases. Second, provided that a patient has been carefully selected, none of the tests or the clinical symptoms alone gives a posttest probability strong enough to make a definite diagnosis of ReA. This is especially true for serology and for HLA-B27 testing, but also for the SpA-specific symptoms such as enthesitis, which are of little value if used alone. An exception to this might be a positive PCR result for the detection of Chlamydia in the joint. Third, in order to make the diagnosis of ReA, a combination of tests or clinical symptoms or both has to be applied, such as clinical evidence of a preceding infection plus a positive result on serology or identification of a bacterium by serology or culture/PCR/LCR plus positivity for HLA-B27 (Figures 1 and 2 and Table 3).