Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (478 page)

   
Antistreptolysin O (ASO)
: ASO antibody testing is the most commonly used and standardized test to diagnose prior GAS infection. Antibody response is brisk after upper respiratory tract infection: detectable antibodies appear approximately 1 week after acute infection and reach maximum titers 3–6 weeks after acute infection. Skin infections (impetigo, pyoderma), however, do not stimulate a good ASO response, so this assay is not recommended for patient evaluation following skin infections. There are several causes for false-positive test results, including multiple myeloma, hypergammaglobulinemia, rheumatoid factor, or infection with group C or G
Streptococcus
.

   
Anti-DNase B
: The anti-DNase B assay is most useful for the evaluation of patients with acute rheumatic fever or glomerulonephritis after impetigo, pyoderma, or other skin infection. Antibody titers are usually detectable approximately 2 weeks after acute infection and reach peak titers 6–8 weeks after infection. Factors causing false-positive ASO titers do not affect anti-DNase B testing, but false-positive anti-DNase B results may be seen in acute hemorrhagic pancreatitis.

   
Streptozyme
: This assay is based on agglutination of RBCs coated with a number of GAS antigens. The reagents have not been well standardized, so lot-to-lot variation has been documented, in terms of both sensitivity and specificity, limiting the value of this testing.

Rapid GAS detection
: Throat swab for rapid direct antigen testing for GAS has a sensitivity of 70–90% compared to culture on SBA; specificity is approximately 95%. Antigen testing may provide results within minutes, but cultures are recommended in children when antigen testing is negative. A positive antigen test result means the patient has GAS pharyngitis or is a GAS carrier.

Molecular diagnostics
: The sensitivity of the Gen-Probe Group A Strep Direct Test is 89–95%, with >97% specificity. Sensitivity of the LightCycler Strep-A realtime PCR assay is approximately 93%, with specificity approximately 98%. The high sensitivity of these molecular assays for detection of GAS pharyngitis obviates culture in direct assay–negative specimens.

Core laboratory
: In patients with PSGN, abnormal urinalysis (RBCs, WBCs, and casts), anemia, decreased total complement, and C3 and/or increased ESR are typical.

TREPONEMAL DISEASE: SYPHILIS

   Definition

Syphilis is a chronic disease caused by infection with the spirochete
Treponema pallidum
, an unculturable spiral bacterium. Syphilis has a global distribution.
Treponema pallidum
is an obligate pathogen of humans; there are no known animal or reservoirs that serve as a source of infection. The disease is transmitted by exposure of a susceptible individual to active lesions of an infected patient or by transplacental transmission. Congenital or neonatal syphilis may be transmitted directly by contact with infectious lesions or by transplacental transmission, which may occur at any time during pregnancy.

   Who Should Be Suspected?

   In venereal syphilis, a local infection, usually manifested as painless ulcer (chancre), forms at the site of inoculation. There is a high concentration of spirochetes in the ulcer exudate. Wide dissemination of organisms occurs during the phase of primary syphilis. Chancres generally heal spontaneously within several weeks.

   Signs and symptoms of secondary syphilis occur several weeks to months after resolution of primary syphilis. The rash of secondary syphilis is most characteristic, typically involving the palms and soles. A wide variety of nonspecific symptoms may also be seen, including fever, malaise, headache, lymphadenopathy, and eye involvement (e.g., uveitis). The symptoms of secondary syphilis usually resolve spontaneously.

   Latent phase: The patient is typically asymptomatic.

   In late (tertiary) syphilis, symptoms related to chronically infected organ systems manifest, most commonly cardiovascular disease (e.g., aortitis), CNS disease (e.g., tabes dorsalis, paresis), and gummatous disease (nodular lesions of the skin, bone, or other tissues).

   Patients with AIDS are at increased risk for severe
T. pallidum
infection.