A Three-Tube Scheme for Same-Day Presumptive Identification of Klebsiella pneumoniae in Clinical Specimens

Abstract

A three-tube scheme that comprised a urease tube, motility tube and a gluconate-nitrate composite medium tube was evaluated for its reliability in correctly predicting the identity of Klebsiella pneumoniae after six hours of incubation. A total of 33 strains were used to assess the scheme, of which 17 were laboratory stock cultures, 14 were fresh clinical isolates, and two were environmental isolates from soil samples. The three tubes were heavily inoculated to give a density approximating McFarland No. 7 turbidity standard, which is roughly equivalent to a bacterial suspension with a concentration of 21 x 108 organisms/ml. The three-tube scheme identified all the five strains of Klebsiella pneumoniae tested correctly, but it was only able to accurately identify 23 out of 28 strains that were not Klebsiella pneumoniae. The scheme falsely identified five test strains that were not Klebsiella pneumoniae. The scheme had a sensitivity of 100 %, a specificity of 82.1 %, a positive predictive value of 50 % and a negative predictive value of 100 %. The scheme should perform better if the distinctive colonial characteristics of isolates on MacConkey agar were considered when predicting making prediction about identity, and a stricter interpretation of the results provided by the scheme was adopted.