The urine minimum inhibitory concentration (MIC) antibiotic sensitivity test methodology was introduced by Antech Diagnostics to improve the predictability and clinical utility of antimicrobial treatment for urinary tract infections. 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 growth of susceptible bacteria is inhibited.

The "susceptibility breakpoint" is an important concept in interpreting MIC results. The susceptibility breakpoint is the concentration of antibiotic slightly greater than that required to kill sensitive strains of bacteria. Many strains of bacteria will be killed by low concentrations of the antibiotic; these are sensitive strains of bacteria. Bacteria with intrinsic or acquired resistance mechanisms typically require much higher concentrations of antibiotic to be killed. The concentrations of antibiotics tested include the breakpoint concentration and 2 to 4 doubling dilutions below the breakpoint. In interpreting MIC test results, the further that the MIC for a particular antibiotic is below its susceptibility breakpoint, the more effective that antibiotic is likely to be against that particular bacterium.

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.

According to the recent ACVIM Consensus Statement, more attention should be given to the detection, evaluation, monitoring, and treatment of dogs and cats with proteinuria. Proper assessment is important to determine the cause of proteinuria and albuminuria. Persistent renal proteinuria usually indicates the presence of chronic kidney disease with its attendant increased risk for adverse health outcomes. [See February 2005 issue of Antech News for detailed review of microalbuminuria (MA)].

The most common causes of MA are LUTD (cystitis, urolithiasis, and neoplasia), and glomerular endothelial or renal tubular damage. In the absence of bacteriuria, gross hematuria, or pyuria, the likely origin of MA is glomerular or renal tubular. Establishing rising or persistent MA (2 or 3 positive tests, at least 2 to 3 weeks apart) is recommended before considering further renal diagnostic work-up. A urine protein:creatinine ratio is indicated if the MA concentration is >30 mg/dL. Other causes of MA include various infectious and metabolic diseases, chronic skin disease, and dental disease.

References: Osborne, Compendium 17:1233-1249, 1995; Meyer and Harvey, Vet Lab Med, 2nd ed, Saunders, Philadelphia, 1998, pp. 221-235; Forrester, et al, J Vet Int Med 13:557-560, 1999; Bruyette, et a., Antech News, April 2001; Aucoin, Antech News, May 2001; Albasan et al, JAVMA 222: 176-179, 2003; Antech News, Feb 2005; Lees et al, ACVIM Consensus Statement, J Vet Int Med 19:377-385, 2005.