Modified Methylene Blue Screening Method for Significant Bacteriuria

Urinary tract infection is common in the tropics especially in third world countries. In most regional laboratories the volume of urine specimens requested for culture and sensitivity are so overwhelming that it poses a great pressure on the laboratory staff and also consumes a lot of reagent. Evidence abound to support the fact that less than 50% of all such specimen are positive hence the need for a screening method that will reduce the large quantity of urine specimen that will undergo culture. The screening for significant bacteriuria will therefore obviate the wastage of materials, reagent, hospital manpower and time. The modified methylene blue screening technique was performed by adding 20 μl of methylene blue stain to 10 ml of well mixed urine sample and allowed to stand at room temperature for 5 minutes before reading the absorbance at 540 nm wavelength against water blank. The absorbance of the sample is compared with that of the cut-off value and those that exceed this value are recorded as being positive for significant bacteriuria while those with absorbance below the cut-off value are recorded as being negative and should not be cultured. The results obtained from the screening technique were compared with semiquantitative urine culture results. A total of 2683 samples were assayed, among this 984 (36.68%) had absorbance above the cut-off value and so were recorded as being positive for significant bacteriuria whereas 1699 (63.32%) had absorbance below the cut-off value and were recorded as being negative for significant bacteriuria. When compared with the semiquantitative culture, a total of 933 (34.85%) had isolates with significant bacterial growth of ≥105 CFU/ml while 1748 (65.15%) had either no bacterial growth or non-significant bacterial isolates. The technique showed a sensitivity of 94.82% and a specificity of 97.17%. The difference in significant bacteriuria between the urinary screening technique and culture isolate was statistically significant (p<0.05). We present our technique for further study while we advocate their adoption in clinical laboratories especially in poor resource setting in developing countries.