Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1396 page)

If the assay is used with cord blood as the specimen source, positive results should be interpreted with caution. The presence of IgG antibodies to mumps in cord blood may be the result of passive transfer of maternal antibody to the fetus. A negative result, however, may be helpful in ruling out infection. Salivary mumps IgM testing is standard in the United Kingdom. The pattern of response and accuracy is very similar to that for serum IgM.

MYCOBACTERIA (AFB, TB) CULTURE

   Definition

   Mycobacteria may cause acute and chronic infections. Infections may be localized or systemic, and there is significant overlap with signs and symptoms of fungal and other bacterial infections. Isolation of mycobacteria requires special culture techniques.

   Mycobacteria are usually acquired via the respiratory route, and the lower respiratory tract is the site of most serious mycobacterial infections.
M
.
tuberculosis
is the most common pathogen associated with these infections. Other mycobacterial species, including other species in the
M
.
tuberculosis
complex and
M
.
avium
complex (MAC), may cause chronic pulmonary infections.

   Organisms may disseminate from the site of primary infection to cause localized or systemic infection. Virtually all organ systems may be involved. The CNS, bone, and urinary tract are common sites of extrapulmonary infection. Mycobacteria may be isolated from stool, most commonly in HIV-infected patients, but the role of mycobacteria as a cause of GI infection has been questioned.

   Superficial mycobacterial infections, such as “swimming pool granuloma” caused by
M
.
marinum
and wound infections caused by rapidly growing mycobacteria, may be caused by direct inoculation of environmental non–
M
.
tuberculosis
species.

   Use

   Mycobacterial culture is used to detect mycobacterial pathogens and to provide isolates for susceptibility testing and further characterization.

   Special Collection and Transport Instructions

   Collect specimens using procedures that minimize the contamination with the patient’s endogenous flora.

   Because routine bacterial, fungal, and other types of infections may be in the differential diagnosis when mycobacterial infection is suspected, ensure that a sufficient volume of infected material is collected to ensure that all testing can be performed.

   For the diagnosis of TB, a minimum of three sputum specimens should be submitted for culture. Patients must be carefully instructed in the proper technique for sputum collection.

   Early-morning specimens are preferred because of pooling of secretions at night. A minimum of 5–10 mL of sputum should be submitted for each specimen.