Current Testing in Infectious Diseases

If Robert Koch or Christian Gram should suddenly reincarnate and wander through a modern microbiology laboratory, they’d feel right at home. Koch, who invented culture media, and Gram, developer of the stain that bears his name, would note little difference in methods from their heyday in the 1880s. Despite the innumerable advances in our knowledge of infectious diseases, we still streak our plates and stain our smears much as they did. But despite the stability in the core technology, there are many ways that the microbiology laboratory has evolved. Keeping up with the changes can be critical in both diagnosis and therapy.

We summarize here some ways that the microbiology laboratory can aid the physician in decision making. Each lab will have its own methods available, and the field is rapidly developing, so be sure to stay abreast of your own facility’s test menus.

Susceptibility Testing

Most laboratories now use automated methods to determine an organism’s sensitivity to a group of antibiotics. Results are reported both as a number, the minimum inhibitory concentration, or MIC, and as an interpretation, sensitive (S), intermediate (I), or resistant (R). The MIC tells us the amount of an antibiotic necessary to inhibit the growth of the organism — the lower the number the more effective it should be. The interpretation is based on usually achievable serum levels of antibiotic in healthy individuals. If usual serum levels differ from other body sites, the interpretation may not be valid. For instance, if a strain of Pseudomonas aeruginosa in a urinary tract infection is susceptible to 4 µg of gentamicin, the organism would probably respond since gentamicin reaches high levels in the urine. But the same strain of Pseudomonas in the lungs may not respond since gentamicin does not achieve the same concentration in bronchial secretions as it does in the blood.

For routine organisms most sensitivity results should be available within two days of submission of the specimen. Proper reliance on the data is essential to good patient management and the judicious use of antibiotics. Physicians should strive to use the least costly, narrowest spectrum antibiotic possible.

Some organisms have not shown resistance to some antibiotics. Hemolytic streptococci, notably groups A, B, C, and G, are uniformly sensitive to the penicillins, and susceptibility testing is not routinely performed. If a penicillin is contraindicated and a macrolide is considered, susceptibility testing should be done.

Antigen Testing

Antigen testing lets us identify a small component of a microbe quickly. A monoclonal antibody attaches to the microbe’s specific antigen, and that complex is detected by a color-producing enzyme. The test is known as an enzyme-linked immunosorbant assay, or ELISA, sometimes called simply EIA. Immunoassays now available for a number of pathogens are listed in Table 1.

Despite its convenience and specificity, antigen testing is never as sensitive as culture. For instance, Group A strep antigen from the throat is picked up in only about 85–90 percent of patients who are culture-positive for the organism. Test specificity is usually very good, but sensitivity may vary 50–90 percent depending on the test. Testing for the presence of an antigen should usually be thought of as a “down and dirty” type of assay, diagnostic if positive, but not the last word.

Table 1. EIA Tests Generally Available: Pathogen • Antigen • Specimen

• Group A Streptococcus • Group A carbohydrate • Throat swab
• Streptococcus pneumoniae • Core polysaccharide • Urine, CSF
• Giardia lamblia • Surface antigen • Stool
• Clostridium difficile • Toxins A and B • Stool
• Influenza A and B • M2 protein • Nasopharyngeal swab
• Respiratory Syncitial virus • Surface protein • Nasopharyngeal washing
• Rotavirus • VP2 and VP6 proteins • Stool
• Legionella • Cell wall lipopolysaccharide • Bronchial washing, sputum

Molecular Assays

Assay by molecular methods such as polymerase chain reaction (PCR) is far more sensitive than EIA testing for antigens. It’s also much more expensive and requires specialized equipment and facilities. The test recovers, amplifies, and identifies the genetic material from pathogens, living or dead. With the recent government mandate for MRSA screening, more laboratories will probably soon be equipped with the rapidly advancing equipment and test methods.

While very sensitive, the testing does have some drawbacks. Susceptibility testing is not available. For some organisms, that doesn’t matter, but for others it can be essential. Most labs prefer tests that are FDA-approved, but the FDA has been very slow to approve commercially available assays. The tests are quite expensive compared to conventional tests, and if test volume is low, they may be batched or sent out, potentially delaying diagnosis.

Table 2 lists molecular tests that are becoming more widely available.

Table 2. Molecular Tests Generally Available: Pathogen • Specimen

• Bordetella pertussis • Nasopharyngeal swab
• MRSA screen • Nasal swab
• Chlamydia trachomatis, Neisseria gonorrhoeae • Cervical, urethral swab
• Gardnerella vaginalis, Trichomonas • Vaginal swab
• Group A Strep • Throat swab
• Group B Strep (prenatal screen) • Vaginal swab
• Acid Fast Bacilli (MTB and MAC) • Sputum, bronchial
• HIV: Quantitation and Ultraquantitation • Blood
• Hepatitis C: detection and genotyping • Blood
• Hepatitis B • Blood
• Herpes simplex virus • Various

Test Availability

Laboratories — in this era of cost consciousness — must depend on reference laboratories for low-volume, time-consuming tests. The higher the test volume, the more likely your lab will perform the test. Discuss any testing needs with your pathologist or laboratory manager.