Indian Journal of Medical Microbiology Medknow Publications on behalf of Indian Association of Medical Microbiology ISSN: 0255-0857 EISSN: 1998-3646 Vol. 25, Num. 3, 2007, pp. 301-302

Indian Journal of Medical Microbiology, Vol. 25, No. 3, July-September, 2007, pp. 301-302

Correspondence

A photometric screening for significant bacteriuria

Bednar M, Nemeckova V

Department of Medical Microbiology, Charles University, 3rd Faculty of Medicine, Prague
Correspondence Address: Department of Medical Microbiology, Charles University, 3rd Faculty of Medicine, Prague
bednarm@cbox.cz

Date of Submission: 18-Apr-2006
Date of Acceptance: 25-Mar-2007

Code Number: mb07086

Dear Editor,

To predict the outcome of urine cultures, several screening methods have been developed. ^[1],[2],[3] In photometric screening, ^[2] diluted urine specimen is added to the broth in microplate well and incubated; if the specimen contains at least 10 ⁵ bacteria/mL, optical density (OD) in the well increases significantly within five hours. The aim of this study was to verify this method using a kinetic microplate reader.

Four hundred thirty midstream urine specimens were tested by the standard culture method. Specimens with counts ≥ 10 ⁵ cfu/mL were considered positive. The specimens were also evaluated using a photometric screening. Urine specimens (100 µL) were inoculated in to 100 µL of brain heart infusion (Oxoid, Basingstoke, UK) enriched with 8% of concentrated tissue culture medium E-199 (Sevapharma Prague, Czech Rep) in microtitre wells. The plate was placed on a photometer (MRX HD; Dynex Laboratories, Chantilly, VA). The temperature of the microplate chamber was maintained at 36°C. The optical density (OD) of inoculated wells was measured every ten minutes at a wavelength of 420 nm. Wells with an OD increase of ≥7% in four hours were considered as positive. Curves of turbidity increase were also received and those that contained an exponential segment were considered positive. The quantitative culture test and photometric screening thus resulted in three logical values: significant/ insignificant bacteriuria; presence/ absence of 7% increase in OD in four hours; and presence/ absence of an exponential segment in curve. Relation among those logical values was expressed as sensitivity and specificity, positive and negative predictive values of the screening.

The results are shown in the [Table - 1]. The exponential character of turbidity increase was less sensitive yet a more specific screening criterion, thus it is suitable for preliminary confirmation of significant bacteriuria. The non-specified turbidity increase estimation ^[2] has a high negative predictive value and is suitable (just like the biochemical screening) ^[1],[3] for preliminary exclusion of significant bacteriuria. Using a microwell plate and a photometer with a kinetic programme, the photometric method can be used for the reliable, rapid and inexpensive screening of bacteriuria.

References

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