Laboratory findings in urinary tract infections.

Urinary tract infection should be considered in a differential diagnosis on the basis of history, physical examination, and urinalysis. To definitively diagnose urinary tract infection, significant bacteriuria must be found by quantitative bacterial culture. The absolute definition of significant numbers of bacteria varies with the method of collection because of the possibility of contamination with the normal bacterial flora of the lower genitourinary tract. Numbers of bacteria are also influenced by the manner in which urine samples are handled, by urine concentration, and by frequency of voiding. Quantitative and qualitative urine cultures should also be used to monitor the efficacy of treatment in chronic and recurrent infections. Cultures should be repeated three to five days after the termination of antimicrobial therapy to ensure elimination of infection. If feasible, cultures should also be repeated two to three days after begining therapy to ensure the antimicrobial agent selected is effective. Remission of clinical signs should not be used to judge efficacy of treatment, especially in chronic or recurrent infections, since infections can persist without causing clinical signs, particularly if bacterial numbers are temporarily reduced. Determination of the minimum inhibitory concentration of an antibiotic for a particular bacteria is preferable to Kirby-Bauer antibiotic sensitivity testing in urinary tract infection because of the difference in serum and urine concentrations of most antibiotics. Bacteria are not sensitive or resistant to an antibiotic but rather to a concentration of that antibiotic. If Kirby-Bauer sensitivity testing is used for urinary tract infection, results must be interpreted carefully since drugs reported as ineffective may be effective in vivo.