However, various laboratory tests and imaging studies can help define the underlying diagnosis
Ankylosing spondylitis: Erythrocyte sedimentation rate (ESR), HLA-B27, sacroiliac joint radiographs
Behçet syndrome: Clinical diagnosis (though imaging of CNS lesions may be suggestive)
Candida infection: Dilated funduscopic examination, blood cultures
Crohn disease: Complete blood count (CBC), ESR, barium upper gastrointestinal series with small bowel follow-through or CT enterography or capsule (video) imaging of small intestine, colonoscopy with biopsy of intestine
Cryptococcus infection: Serum cryptococcal antigen; cerebrospinal fluid (CSF) for opening pressure, white blood count (WBC), glucose, protein, cryptococcal antigen, culture; blood culture; MRI or CT of head
CMV infection: CMV antibodies, polymerase chain reaction (PCR)
Herpes simplex virus infection: Viral culture and direct fluorescent antibody of vesicular skin lesions
Herpes zoster virus infection: Viral culture and direct fluorescent antibody of vesicular skin lesions
HIV infection: HIV-1 antibody ELISA, confirmed by Western blot, absolute CD4 lymphocyte count
Mycobacterium tuberculosis: Tuberculin skin test, cultures of sputum, pleural fluid, CSF
Psoriasis: Clinical diagnosis based on typical skin lesions, nail pitting or onycholysis
Reactive arthritis: Clinical diagnosis
Syphilis: Nontreponemal antigen tests (serum rapid plasma reagin [RPR] or Venereal Disease Research Laboratory [VDRL]), treponemal antibody tests (enzyme immunoassay [EIA] or chemiluminescence assay [CLIA]) and T pallidum hemagglutination ([TPHA], T pallidum particle agglutination [TPPA]), and treponemal antibody absorption [FTA-ABS] tests, PCR of genital ulcers, CSF for protein, WBC, VDRL
Toxoplasmosis: Isolation of T gondii or identification of tachyzoites in tissue or body fluids; PCR of aqueous humor, CSF, blood, other body fluids; serologic tests (including ELISA, indirect fluorescent test, agglutination test) for Toxoplasma antibodies; CT or MRI scan of head
Ulcerative colitis: CBC, ESR, serum albumin; sigmoidoscopy