Use of External Quality Assessment to Evaluate the Reliability of Quantitative Urine Culture

A clinical diagnosis of urinary tract infection is made by physicians. However, the microbiological diagnosis is based on detecting the growth of uropathogens in numbers considered significant. To evaluate how reliably the laboratories are able to perform the quantitative culture of simulated urine specimens, we analysed 6932 External Quality Assurance (EQA) results of 335 laboratories. All laboratories participating in EQA were licensed according to Finnish regulations and used standardised methods. Of all EQA reports, the numbers of bacteria were correct in 83% of results: 91% of cases of gram-negative bacteria (93% of Escherichia coli results and in 80% of Klebsiella sp. results) and 68% for gram-positive bacteria results (84% for Enterococcus sp. and 23% for Streptococcus agalactiae). The correct number (i.e. correct result) was significantly associated with the culture method used (dipslide 69%, plate 86%; p<0.001). The correct number of Enterococcus sp. was reported by 45% of the dipslide and 94% of the plate users. The corresponding percentages for S. agalactiae were 63% and 96%. There was an interaction between bacteria present in the specimen and the method used, although the correct result was more dependent on the bacterial group (grampositive or gram-negative) than the method. Based on these EQA results, more education is still needed for the laboratories to interpret the results of routine urine culture and that special urine culture with wider selection of plates should be available to detect the less common uropathogens. Our results also show the importance of close contact with the physicians and the diagnostic laboratory, so as to be well informed of the culture method (dipslide vs. plate) used and to give the laboratory information required, thus, being able to make a correct diagnosis of urinary tract infection in a patient.