Urinalysis

The examination of a patient's urine is important in screening for disease, and forms part of the complete physical examination of any patient. It is often performed outside the laboratory by non-laboratory staff. Although the testing of urine is relatively simple, failure to follow the correct procedure may lead to inaccurate results.

A midstream clean-catch technique is usually adequate. If the urine cannot be examined promptly, it should be refrigerated. Delays of more than two hours between collection and examination often cause unreliable results.

Physical properties: color and odour

Foods, medications, breakdown products and infection can cause abnormal urine color. A few examples are:

• brown: bile pigment, Metronidazole (antibiotic)
• green/blue: urinary tract infection, Amitriptyline, cimetidine
• red: blood in urine, beets, Rifampicin (anti-TB drug)
• yellow: concentrated urine, carrots

The normal odour of urine can be strong, but does not imply infection. Diabetic ketoacidosis can cause urine to have a fruity odor, prolonged urine retention can cause a ammonia smell, urine from a patient with a urinary tract infection (UTI) usually smells pungent, and certain foodstuffs can give urine a distinctive smell (for example asparagus).

Biochemical testing

Biochemical testing of urine involves the use of commercially available disposable strips, impregnated with a number of coloured reagent blocks. When the strip is manually immersed in the urine specimen, the reagents in each block react with a specific component of urine in such a way that the block changes color if the component is present. The color change produced is proportional to the concentration of the component being tested for.

When using these disposable strips, it is important to make sure that the strips have not expired, that they are stored properly and that the manufacturer's instructions are carefully followed - if the test strips are read too quickly, or left too long, they can give inaccurate results.

The range of components routinely tested for includes:

• Glucose
• Glycosuria indicates that the filtered load of glucose exceeds the ability of the renal tubules to reabsorb all of it. This usually points to a high blood glucose and should prompt a formal test for diabetes mellitus. Glycosuria is not always due to diabetes and may also occur in pregnancy, where the renal threshold for glucose may be lowered.

• Bilirubin

Urine does not normally contain bilirubin - its presence should prompt further evaluation for liver disease and biliary tract obstruction (for example gallstones).

• Urobilinogen

Normal urine contains only small amounts of urinoblilinogen. High levels can indicate haemolysis (breaking of red blood cells) and liver disease.

• Ketones

Ketones in the urine usually indicate that the body is using fat to provide energy. This can occur in uncontrolled diabetes mellitus or with prolonged fasting.

• Specific gravity

Specific gravity reflects urine concentration and a patient's hydration status.

• pH

The urine is usually acidic. It can help to determine if the kidney is handling acid appropriately and assists in the investigation renal stones.

• Protein

Proteinuria may signify abnormal excretion of the proteins by the kidneys and persistent proteinuria should be investigated further. It is important to note that test strips are sensitive to albumin and may not detect other proteins and may be falsely positive if the urine is too concentrated, too alkaline or contaminated with blood or semen.

• Blood (haematuria)

Haematuria may be due to contamination from menstruation, exercise-induced, UTIs, kidney stones and cancer. Exercise-induced haematuria should clear up within 72 hours.

• Nitrite

Positive result points towards a urinary tract infection. Incorrectly stored reagent strips may give false positive results.

• Leucocytes

Leucocytes in the urine suggest a urinary tract infection. Other causes include tuberculosis, exercise, kidney stones and bladder tumors.

Microscopic urinalysis

Microscopic examination is an indispensable part of urine examination; the identification of casts, cells, crystals, and bacteria aids in the diagnosis of a variety of conditions.

Take-home message

• Urinalysis is an important part of the clinical examination of every patient
• Chemical analysis of a urine specimen is carried out using commercially available disposable strips. These strips can, when stored incorrectly, have expired, or if read at the incorrect time, give unreliable results
• All persistently abnormal test strip results need to be followed up with a more formal, confirmative laboratory test