No specific diagnostic test is available. The diagnosis is established by fulfillment of the clinical criteria (see Clinical Manifestations) and clinical or laboratory exclusion of other possible illnesses, such as measles, parvovirus B19 infection, adenovirus or enterovirus infections, staphylococcal or streptococcal toxin-mediated disease, rickettsial exanthems, drug reactions (eg, Stevens-Johnson syndrome), leptospirosis, systemic onset juvenile idiopathic arthritis, and reactive arthritis. A greatly increased ESR and serum CRP concentration during the first 2 weeks of illness and an increased platelet count (greater than 450 000/mm³) on days 10 to 21 of illness are almost universal laboratory features. ESR and platelet count usually are normal within 6 to 8 weeks; CRP concentration returns to normal much sooner.