Dark-field Microscopy

 

Microscopic examination of the centrifuged urine using dark-field illumination is considered to be a convenient and rapid diagnostic test. Midstream urine sample is collected in a sterile bottle in a very hygienic condition. It has been reported that the rate of destruction of leptospires by urine and products of tissue autolysis is more rapid during storage at 20C than 4C (Miller et al., 1990). So all clinical samples including urine should be placed in an ice-container in the dark, and transported to the laboratory as soon as possible.

However, dark-field microscopic (DFM) examination is insensitive and requires a skilled observer to differentiate leptospires from artefacts. It should be considered that leptospires are very fragile and sensitive to the pH of the urine, therefore, for maximum efficiency, one-half of each urine sample should be submitted with added formalin (1 drop to 20-30 mL of urine) and the other half submitted in the fresh state. The formalin prevents bacterial overgrowth and the fresh urine sample may be used for culture.