Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1365 page)

   
Negative results:
The risk of early-onset GBS neonatal infection is markedly reduced but not eliminated in patients with negative vaginal–rectal carriage screens.

   Limitations

   GBS carriage may be intermittent and may fall below the level of detection at the time of screening culture. Optimal detection depends on the quality of specimen collection. Both rectal and lower vaginal specimens should be submitted. Specimens unacceptable for culture: cervical, perianal, perirectal, and perineal specimens; a speculum should not be used for collection. Nonhemolytic GBS may be missed if colonies with consistent morphology are not ruled out by additional specific testing.

Suggested Reading

Prevention of Perinatal Group B Streptococcal Disease. Revised Guidelines from CDC, 2010.
MMWR
November 19, 2010;59(RR-10).

HELICOBACTER PYLORI
SEROLOGY SCREEN (
H
.
PYLORI
ANTIBODY [IgG, IgA, AND IgM] SCREEN)

   Definition

   
H
.
pylori
is a bacterium that is found in the stomach of about two thirds of the people in the world, although most infected people will never develop disease.
H
.
pylori
infection is a major risk factor for peptic ulcer disease. These bacteria are responsible for the large majority of gastric ulcers and upper duodenal ulcers. Research studies indicate that infection with
H
.
pylori
increases the risk of gastric cancer, gastric mucosa-associated lymphoid tissue (MALT) lymphoma, and possibly pancreatic cancer.

   
Normal range:
Negative.

   Use

   Screening for
H
.
pylori
infection.

   Laboratory-based serologic testing to detect
H
.
pylori
IgG antibodies is inexpensive and noninvasive. This is the predominant serologic test available for clinical use, and it is well suited to primary care practice. However, concerns over its accuracy have limited its use. Large studies have found that it has high sensitivity (90–100%) but variable specificity (76–96%); the accuracy has ranged from 83 to 98%.

   Some studies found that IgA antibodies may detect cases that were negative by IgG testing. However, a number of studies have demonstrated that IgA testing is overall less sensitive and less specific than IgG testing. Some laboratories also offer IgM tests, which if elevated would indicate an acute infection. IgM assays have little or no role in clinical practice for the diagnosis or management of what is almost always a long-standing condition by the time
H
.
pylori
infection is considered.

   Interpretation