Acute onset of rash and oligoarthritis

Test blood and urine; check the ankles

Although there is no specific test for RS, several laboratory procedures are essential to honing in on the diagnosis. Hematological inquiry will confirm anemia, leukocytosis, thrombocytosis, and an elevated erythrocyte sedimentation rate (ESR). Though the urethral test may not be positive for a suspected organism, this procedure must be done to rule out gonococcal or chlamydial infection. This can now be done on a urine specimen rather than inserting a swab into the urethra. The urine is sent for a polymerase chain reaction (PCR) test rather than a culture. If enteritis was the preceding infection, a stool culture to elucidate potential pathogens is warranted.

You’ll also need to order serological tests for antinuclear antibodies (ANAs), rheumatoid factor (RF), and HIV. As you would expect, these tests will be abnormal for systemic lupus erythematosus, rheumatoid arthritis, and HIV respectively. Though these tests are often negative in RS patients, a strong association with HIV infection does exist.

Keep in mind, too, that you can differentiate gonococcal arthritis from RS based on historical features, as well as clinical features, including migratory polyarthritis with necrotic and pustular skin lesions. Patients with gonococcal arthritis will also have a positive gonococcal culture and rapid improvement with antibiotics.

If you order a biopsy, pathology is likely to find a variety of features in an RS patient, such as spongiform pustules, neutrophilic infiltrate in a perivascular pattern, and an epidermal hyperplasia that resembles psoriasis.³

Radiographic imaging for a suspected case of RS may reveal a number of signs that resemble psoriatic arthritis (pencilin-cup deformity, syndesmophytes, sacroiliitis), but enthesitis, particularly in the ankle joint region, should raise your index of suspicion for RS.⁶