count the number of colonies growing on each plate. Multiply the number of colonies by the dilution factor to estimate the bacterial load in the original urine specimen For example, if you counted 50 colonies while using the 0.01 mL inoculating loop, the calculation is 50 x 100 = 5,000 CFU/mL, with 100 being the dilution factor. If you used the 0.001 mL loop to do the streaking, the colony count is calculated as: 50 (colonies) x 1,000 (the dilution factor) = 50,000 CFU/mL.
After you have gained experience, you can usually estimate 100 or greater colonies by cursory inspection of the plate without actually performing an accurate count. Confluent growth generally is considered to be in the range of 1,000 colonies, a number that is then multiplied by the dilution factor. Traditionally, bacterial colony counts have been reported in the following categories:
*less than or equal to 10,000 CFU/mL
* About 50,000 CFU/mL
* greater than or equal to 100,000 CFU/mL
A colony count of greater than 100,000 CFU/mL generally indicates a UTI; counts of less than 10,000 CFU/mL usually represent contamination. The significance of counts in the range of 50,000 CFU/mL must be determined by the physician, based on clinical and other laboratory information.