May • 2002

BENEFITS OF URINE MIC TESTING

To improve the predictability and clinical utility of antimicrobial treatment for UTI, Antech Diagnostics recently introduced the urine minimum inhibitory concentration (MIC) antibiotic sensitivity test which uses an automated microdilutional technique.

MIC testing guides antibiotic selection by determining whether the tested bacteria are likely to be sensitive or resistant to any given antibiotic.

Most antibiotics are tested over 3-5 doubling dilutions that represent a range of concentrations (in µg/mL) at which susceptible bacteria are either inhibited or killed and can be achieved in urine and serum following standard dosing regimens.

The concentration of antibiotic slightly greater than that required to kill sensitive strains of bacteria is called the susceptibility break point.

Key points in using the urine MIC panel to choose an antibiotic and dose:

• All other things being equal (cost, safety, ease of use, compliance), use the antibiotic with the lower MIC in its testing range.
• The higher the MIC in the testing range, the higher the dose needed and vice versa.

References: Forrester, et al., JVet Int Med 13:557-560, 1999; Bruyette, et al., Antech News, April 2001; Aucoin, Antech News, May 2001.

IONIZED CALCIUM MEASUREMENT Ionized calcium must be run on serum not previously exposed to air. Samples for ionized calcium or the PTH/ionized calcium combination test should be submitted in a spun and frozen SST, and sent to the laboratory on cold packs. Do not decant the serum. Serum that has been separated and transferred to a transport tube is not suitable for ionized calcium testing, because the measured concentration is falsely decreased in such samples. If other tests in addition to ionized calcium or PTH/ionized calcium tests are required, please submit a second serum sample.

COAGULATION SCREENING TESTS & ACTIVE BLEEDING During acute bleeding crises, the patients typically will demonstrate an "acute-phase" response which will elevate fibrinogen concentration, may elevate D-dimer, and will shorten the clotting times in PT and APTT assays. Platelet numbers may be high, normal, or low, and leukocytes are usually elevated. Therefore, in cases with significant bleeding, such as young hemophiliacs, the high fibrinogen will shorten the PT and APTT endpoints. The APTT, which is expected to be prolonged in hemophilia, may then be only slightly elevated (1-5 seconds above upper reference limit) or even within the upper end of reference range, masking the true diagnosis. This effect, along with the age, sex, and clinical history, must be considered when evaluating the PT and APTT from animals suspected of having coagulopathies. Specific clotting factor assays are needed to confirm the diagnosis.