Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1313 page)

Rubella Serology Screen (Rubella IgG and IgM)

Sexually Transmitted Infections, Molecular Diagnosis (
Chlamydia trachomatis
,
Neisseria gonorrhoeae
,
Trichomonas vaginalis
)

Sputum Culture (Routine)

Staphylococcus aureus
(SA) and Methicillin-Resistant
Staphylococcus aureus
(MRSA)

Stool Culture (Routine)

Streptococcus, Group A, Direct Detection (Antigen, Nucleic Acid)

Streptococcus pneumoniae
Urine Antigen Test

Streptozyme, Antistreptococcal Antibodies, Antistreptolysin O [ASO], Anti-DNase-B [ADB]

Syphilis Serology Tests

Throat Culture (Routine)

Throat and Pharyngeal Culture, Patients with Cystic Fibrosis

Toxoplasma Serology Screen (
Toxoplasma gondii
, IgG and IgM)

Trichomonas vaginalis
Molecular Detection

Urine Culture (Routine)

Vaginitis Panel, Molecular Probe

Vancomycin-Resistant Enterococcus (VRE) Screen Culture

Varicella-Zoster Virus (VZV) Culture (Rule Out)

Varicella-Zoster Virus (VZV) Direct Detection (DFA)

Varicella-Zoster Virus (VZV) Serology Screen (IgG and IgM)

Vibrio
Culture of Stool (Rule Out)

West Nile Virus (WNV) Serology

Wound Culture

Yersinia enterocolitica
Culture (Rule Out)

This Chapter presents the most commonly ordered infectious diseases–related tests arranged in alphabetical order. For pathogen-specific tests, the entry is grouped under the pathogen name. The test methods in this Chapter include general cultures by source, targeted cultures to rule out specific pathogens, direct antigen assays, antibody assays, macroscopic and microscopic detection, as well as molecular methods.

It is important to consider the limitations for each of these methods. In general, cultures are considered the gold standard for pathogen detection; however, testing typically takes 24–48 hours for completion and longer for fastidious pathogens (like anaerobes) or slow growing organisms (like mycobacteria). Targeted cultures are available and requested when pathogens cannot be efficiently detected by routine culture methods. Nevertheless, it is important to consider that (1) selective culture conditions may also inhibit some individual strains of the target pathogen; (2) nonselective media should also be inoculated along with selective media; and (3) cultures for specific pathogens may not detect other significant pathogens when used for evaluation of infected patient samples. Submission of routine bacterial cultures appropriate for the specimen type is usually recommended in addition to targeted cultures.

Direct antigen detection assays are widely available and provide quick turnaround time; however, they often have low sensitivity. Molecular methods are becoming common in detecting pathogens due to the high sensitivity and shorter turnaround time than conventional cultures, yet these assays are currently costly. See Chapter
11
for additional information regarding the basis of microbiology identification methods and pathogen-specific diseases.

ACID-FAST BACILLUS (AFB) SMEAR

   Definition and Use

   Smears of patient specimens are stained and examined for the presence of mycobacteria. They may provide early evidence of TB or other mycobacterial diseases. Certain dyes bind to the thick, mycolic acid–rich cell walls of mycobacteria. The lipids of the cell wall make the cells resistant to decolorization with acid–alcohol solutions. AFB staining should be undertaken for most specimens that are submitted for mycobacterial culture.

   
Methods:

   There are two types of AFB stains: chromogenic (carbol-fuchsin stains [hot: Ziehl-Neelsen; cold: Kinyoun]) and fluorogenic (auramine O + rhodamine). After staining, the smear is destained with an acid–alcohol solution, typically HCl in ethanol. Mycobacteria retain the stain.

   In chromogenic methods, slides are examined using a 100× oil immersion objective with light microscopy. Nonmycobacterial cells are counterstained with methylene blue. Mycobacteria appear red, whereas other bacteria and background are stained blue.

   In fluorogenic methods, auramine-stained slides are examined by fluorescence microscopy using a 25× or 40× objective. Mycobacteria are yellow-orange against a dark background. The improved signal-to-noise of auramine fluorochrome staining, allowing scanning with lower power objective, results in examination of a greater area of the slide at a given time, and therefore greater sensitivity. Any detected organisms should be confirmed by examination for typical morphology using the 100× objective. Some laboratories confirm positive fluorochrome smears with a carbol-fuchsin–based stain.

   Specimens should be collected and transported to the laboratory according to recommendations for mycobacterial cultures.

   
Turnaround time:
<24 hours