Microbiology FAQ’s

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Q: What do the numbers on the sensitivity report represent?

A: The minimum inhibitory concentration (MIC) is the lowest concentration of the antibiotic which will inhibit bacterial growth in vitro. The MIC is correlated with the concentration of the antibiotic in blood. The lower the MIC, the more sensitive the organism. However, MICs between different classes of drugs (i.e., aminoglycosides versus penicillin) are not necessarily equal. Consequently, the physician must take into consideration more than just the MIC: site of infection, desired route of administration (oral, parenteral), least toxicity to host, most economical drug, a patient’s allergies kidney function.

On the sensitivity report, DLS also reports the NCCLS (National Committee for Clinical Laboratory Standards) interpretations of “S” (Susceptible), “I” (Intermediate), or “R” (Resistant) as a guide to physicians.

Q: Why is susceptibility testing not done on all organisms?

A: Some organisms (i.e., Beta strep) have predictable sensitivities; always sensitive to Penicillin. Some organisms, (i.e., Diphtheroids, Lactobacillus, Bacillus species, Strep. viridans, and certain isolates of Staph coagulase negative) are considered “usual flora” of the body site and are not tested. For certain organisms, no standardized testing methods are available.

Q: How should a patient prepare for occult blood testing on stool?

A: Patient should not receive vitamin C (ascorbic acid) for 3 days prior to occult blood testing by guaiac. Alcohol and aspirin, especially together, and other gastric irritants (steroids, rauwolfia derivitaves, all non-steroidal anti-inflammatory drugs, colchicine) should also be avoided.

A high-bulk diet free of meat with restriction of peroxidase-rich vegetables (turnips, horseradish, artichokes, mushrooms, radishes, broccoli, bean sprouts, cauliflower, apples, oranges, bananas, cantaloupes, grapes) has been recommended for 72 hours prior to and during guaiac testing to decrease the incidence of false-positives.

Antacids may cause false-negative guaiac tests. Therapeutic iron causes false-positives with guaiac tests in 50% of healthy subjects.

Halogens and cimetidine can cause reactions with guaiac tests.

Q: How should a specimen be collected for occult blood testing on stool?

A: The specimen should not be collected after a barium enema or be contaminated with urine. Place specimen in the appropriate container and take care not to get specimen on the outside of the container.

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