You are working in a hospital lab and receive a urine sample to test for UTI inf

Question

You are working in a hospital lab and receive a urine sample to test for UTI infection. A UTI is diagnosed if the samplehas more than 100,000 CFU/ml. First you must determine the number of bacteria in the sample. You decide to usespectrophotometer, an indirect count method, and then estimate the CFU based on a known conversion factor. Youmeasure the OD (optical density), also called the absorbance, at 600 nm and get a reading of 0.9. To convert to CFU/mlyou will use the conversion factor OD of 1.0 is equivalent to approximately 106 CFU/ml.Calculate the estimated CFU/ml in your urine sample? Would you say that the patient has a UTI?

Explanation / Answer

Answer:

Based on the given information, a UTI is diagnosed if urine sample has > 100,000 CFU/ml

Reading of spectrophotometer (absorbance at 600nm) = 0.9

Conversion factor for Optical density (OD) 1 unit OD = 106 CFU/ml

Since the OD of the urine sample is 0.9, therefore the urine sample contains = OD x (no. of CFU/ml per 1 unit OD)

= 0.9 x 106 = 9 x 105 CFU/ml = 900,000 CFU/ml

Therefore the urine sample contains 9 x 105 CFU/ml. Since the sample is diagnosed for UTI infection if the sample has more than 100,000 CFU/ml and our sample contains around 9 times more CFU/ml than the reference limit, therefore this sample is positively diagnosed for UTI infection.

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