Abstract

Objective: The aim of this study was the evaluation of the efficacy of examination of urine samples by Gram staining and MALDI-TOF MS system.

Materials and Methods: A total of 251 out of 555 urine samples sent for culture to Sisli Hamidiye Etfal Training and Research Hospital Clinical Microbiology Laboratory between September 14 to September 18, 2015 were randomly selected and included in the study. Detection of pure bacterial growth of ?105CFU/ml was aimed by using Gram staining and MALDI-TOF MS. Urine samples yielding at least one bacterium in at least five microscopical fields by Gram staining were tested by MALDI-TOF MS (Bruker Daltonics, Germany). Urine culture was performed by using “Chromagar Orientation” medium (BD, USA). Culture, Gram staining and MALDI-TOF MS results were compared.

Results: In 28 (11.1%) of the 251 samples tested, bacterial growth of ?105 CFU/ml was observed. While single bacterial agent was isolated in 27 of 28 samples, one sample yielded two different bacterial species. In 25 of 28 samples, bacteria with a single morphology was observed with Gram staining. MALDI-TOF MS could identify the bacterial agents in 18 of these 25 samples. While bacterial growth of ?105 CFU/ml wasn’t observed in six samples yielding bacteria with a single morphology by Gram staining, contamination was observed in three samples. When MALDI-TOF MS results were investigated for these nine samples, it was observed that MALDI-TOF MS gave a result of “Escherichia coli” in one of the contaminated samples and no result was obtained in the remaining eight samples. When the culture is considered as the gold standard, three false-negative and nine false-positive results were observed by Gram staining while 10 false–negative and one false-positive results were observed by MALDI-TOF MS. The sensitivity, specificity, and positive and negative predictive values were 89.2%, 95.9%, 73.5%, and 98.6%, respectively, for Gram staining, and 64.2%, 99.5%, 94.7%, and 95.6%, respectively, for MALDI-TOF MS.

Conclusions: Combined use of Gram staining and MALDI-TOF MS in the direct testing of urine samples has allowed bacterial identification for more than half of the samples yielding bacterial growth of ?105CFU/ml in culture,within two hours after receipt of the samples in the laboratory, with high specificity, and positive and negative predictive values. The use of MALDI-TOF MS with higher sensitivity rates will be useful in the diagnosis of urinary tract infections.

Key words: Gram staining, MALDI-TOF MS, urine