We pretreated with SDS 71 urine samples with bacterial counts of >10 5 CFU/ml and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) identification scores of <2, in order to minimize failure rates. Identification improved in 46.5% of samples, remained unchanged in 49.3%, and worsened in 4.2%. The improvement was more evident for Gram-negative (54.3%) than for Gram-positive (32%) bacteria. Urinary tract infections (UTIs) are among the most common human bacterial infections (1).Tests developed for UTI screening include urine dipstick testing, urinalysis, and Gram staining. The urine culture remains the "gold standard," but the use of this method, without any previous screening procedure, is time-consuming and expensive, because of the high cost of unnecessary testing of negative samples (2).Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) has been shown as a fast and reliable method for bacterial identification both from culture plates (3,4) and from blood cultures vials (5, 6), and also from some other samples, such as infected urine, especially when Gram-negative bacteria with high bacteria counts are involved (7).We described recently a procedure for processing urine samples (7) which begins with a centrifugation step (2,000 ϫ g for 30 s) to remove leukocytes. Thus, the 7 to 8% of identification failures reported in culture-positive samples might be associated with the removal of intraleukocytic microorganisms in these first steps of processing. We have developed a further study, including a sample pretreatment with SDS, because this compound can lysate cells and then release microorganisms, thereby increasing method sensitivity.Processing of urine samples. We analyzed 71 urine clinical samples with bacterial counts of Ն10 5 CFU/ml on blood agar after 18 h of incubation in an aerobic atmosphere at 37°C and with a score of Ͻ2 in MALDI-TOF MS identification. Samples showing mixed cultures were discarded.Routine urine samples were initially processed for direct microorganism identification with MALDI-TOF MS, according to previously reported methods (7). An aliquot of each sample was stored at 4°C. When the bacterial count was Ն10 5 CFU/ml and the MALDI-TOF MS identification score value was Ͻ2, the stored aliquot was spread again on blood agar for checking that bacterial count had not changed significantly (count modifications of Ͻ5% were considered acceptable) and processed again for MALDI-TOF MS identification, after SDS pretreatment, always before 24 h of storage. Comparison between improvement rates in Gram-positive and Gram-negative bacteria was performed by using the Fisher exact test with the mid-P method. Statistical significance was considered when the P value was Ͻ0.01. Differential procedure. (i) MALDI-TOF MS.Samples were processed as described before (7). Briefly, urine (3 ml) was centrifuged at 2,000 ϫ g for 30 s to remove leukocytes. The supernatant was centrifuged at 15,500 ϫ g for 5 min to collect bacteria. The pellet was washed once wit...

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