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1.2 Urinalysis and microscopy Oxford Textbook of Clinical Nephrology

1.2 Urinalysis and microscopy
Giovanni B. Fogazzi and Domenico Fenili
Physical and chemical examination Preanalytical aspects Collection of specimens Storage of specimens Physical features Colour Turbidity Odour Density Chemical features pH Haemoglobin Glucose Proteins Leucocyte esterase Nitrites KetonesBilirubin–urobilinogen Enzymuria and brush-border antigens in the urine Urine microscopy Historical background Urine collection and handling Microscopic examination The formed elements The urine sediment of the normal individual The interpretation of urine sediment findings Chapter References

Physical and chemical examination
Preanalytical aspects According to the principle that ‘the analysis is never better thanthe specimen’ (Lalla et al. 1990), the preanalytical phase is an important aspect of urinalysis because the timing of collection, the preparation of the patient, and the storage of specimens can affect the accuracy of the results.
Collection of specimens

Before collecting a specimen of urine, the hands and the external genitalia should be

cleaned with lukewarm water, avoiding the use ofdisinfectants. In females the urethral meatus is best cleaned after spreading the labia and in males after uncovering the glans. Although more scrupulous techniques are often recommended, we believe that these simple procedures are adequate for satisfactory collection of urine. Depending on the type of investigation, a timed urine collection may be needed (for example, 24 h for proteinuria) or aspot urine. In the latter it is preferable to collect the first morning urine, which is the most concentrated and acidic due to the overnight fast. Urine should be collected in clean, but not necessarily sterile, containers. They must not be contaminated by disinfectants, such as quaternary ammonium compounds for instance, which can interfere with the dipstick evaluation of proteinuria.
Storage ofspecimens

To avoid alterations in physical or chemical features, urine should ideally be analysed within 1 h of micturition (Lalla et al. 1990). Several means of preservation, such as thymol, borate, formalin, toluene or refrigeration at 4°C, have been proposed, which, however, can interfere with some chemical reactions. For instance, formalin may precipitate protein and thymol interferes withthe acid precipitation test for proteins (Graff 1983). Thus, there is no definitive substitute for the study of fresh urine. Physical features
Colour

The colour of the urine must be evaluated in optimal conditions, under good lighting with the specimen in a transparent container and viewed against a white background. The colour of normal urine ranges from pale to dark yellow and amber,depending on the concentration of urochrome. Gross haematuria is the most important and frequent cause of altered colour. In this condition the urine is pink to black, the factors that influence the final colour being the number of erythrocytes or the amount of haemoglobin, the pH, and the duration of contact between the haemoglobin and urine. The lower the pH and the longer the contact, the darker thecolour (Berman 1977). Urine is also of a variable red colour in haemoglobinuria, myoglobinuria, after eating beets by some genetically susceptible people, or after rifampicin, conditions that may be suspected in the absence of erythrocytes on microscopy. Jaundice and all other states associated with increased concentrations of conjugated bilirubin are also frequent causes of hyperpigmented urine(dark yellow to brown). In other conditions, urine may be normal in colour when fresh, but darkens upon standing. This occurs in porphyria because of urinary porphobilinogen, in melanoma because of melanogen, and in alkaptonuria because of homogentisic acid. Therefore, when such diseases are suspected the urine must be exposed to light for some time. Drugs may also influence the colour of urine....