Urinalysis is the generic term for all those clinical tests that involve physical, ­microscopical, chemical or microbiological examination of urine. Microbiological examination of urine is the subject of Chapter 21. Another aspect of urinalysis, dipstick testing of urine, is the focus of this final chapter. In contrast to all other tests considered in this book, dipstick testing of urine is almost always performed outside the laboratory, in clinics, on hospital wards and primary care surgeries by non-laboratory staff, usually nurses; it is a ‘point of care test’.

The value of the urine dipstick test lies in its simplicity and potential to screen for serious renal, urological and liver disease as well as metabolic disorders, such as diabetes. A positive result on dipstick testing is never sufficient to make a definitive diagnosis, though it may provide supportive evidence of one. More often a positive result is used to inform what the next step in the diagnostic ­process should be.

After a general consideration of sample collection and what can be learned from a physical examination of urine, attention will turn to the dipstick test itself. Each of the urine constituents tested for will be considered in turn, highlighting the most common pathological and non-pathological causes of abnormal results.

Table 25.1 Changes that occur in urine after voiding.

If a delay in testing is unavoidable, the sample should be kept in a refrigerator, as reduced temperature slows the rate of these changes.

Urine sample collection

The ideal urine sample for urinalysis, including dipstick testing, is a sample of uncontaminated bladder urine. The ‘clean catch’ mid stream urine collection ­technique (Table 21.1) was designed for collection of such an ideal specimen. In practice initial screening can be performed on a midstream urine sample collected without the exacting requirements of a ‘clean catch’ technique, although a chemically clean and sterile container is essential. The manner of sample collection is less important than its freshness. Urine should be dipstick tested within a few hours of voiding, because many urine constituents are unstable. The major changes in urine composition that occur after voiding are summarised in Table 25.1.

Physical inspection of urine

Inspection of urine should focus on: colour, clarity and odour. In health, freshly voided urine is usually a clear, pale yellow/straw coloured fluid, with only slight, if any, odour. Urine appearance and odour can change if left to stand for more than a few hours so that it is important that freshly voided urine is used for physical inspection.

Colour

The pale yellow colour of urine is due to the presence of the pigment urobilin, a product of bilirubin metabolism. In health the variability in intensity of this yellow reflects varying urine concentration; the more concentrated the urine, the darker is the shade of yellow. The main physiological determinant of urine concentration is fluid intake. Overnight urine reflects a prolonged period without any fluid intake, so that the urine passed first thing in the morning is usually the most concentrated and therefore the strongest coloured. Dehydration, whatever its cause, is associated with production of a concentrated urine, that is consequently relatively dark in colour. After heavy fluid intake (particularly alcohol, which has diuretic property), a dilute urine is passed, which may be almost colourless.

Quite apart from these normal physiological changes in colour, urine may be abnormally coloured due to the presence of abnormal urine constituents^1,2. These may be endogenous, in which case the abnormal colour reflects a pathological condition (e.g. bilirubin turns urine dark yellow, blood and Hb turns urine pink/red) or they may be exogenous in origin, simply reflecting ingestion of certain foods (e.g. acidic urine may be red after eating beetroot) or more frequently, prescribed drugs.

Clarity/turbidity

Freshly voided urine is usually clear, but urine from healthy subjects may become cloudy if left for more than a few hours due to precipitation of phosphates as urine becomes more alkaline, or precipitation of uric acid if urine is acidic. The most common pathological cause of increased urine turbidity is urinary tract infection (UTI). In such cases turbidity is due principally to the presence in urine of the causative bacteria and white cells (leucocytes), recruited to fight the infection.

Odour

Ingestion of certain foods (e.g. asparagus, aromatic spices) can affect the odour of urine. Bacterially infected urine often smells characteristically fishy, particularly if left to stand for more than a few hours. Certain species of bacteria (Proteus, Klebsiella and Pseudomonas) elaborate an enzyme, urease, that acts on urea present in urine to produce ammonia. Urine infected with these bacteria has a strong ammoniacal odour. A few metabolic disorders are associated with urinary excretion of odorous chemicals. The most significant of these is acetone excreted in the urine of diabetics suffering diabetic ketoacidosis.

Dipstick testing

Technical considerations

Commercially available reagent strips for urine dipstick testing consist of an inert plastic strip on which is mounted paper pads impregnated with chemical reagents. Each pad contains the chemicals needed for detection of a particular constituent of urine. The urine constituents that can be tested for using commercial dipsticks ­varies according to the particular product being used. Urine constituents detectable by commercial dipstick testing are:

• Glucose
  
• Bilirubin
  
• Ketones
  
• Blood
  
• Leucocyte esterase
  
• Nitrite
  
• Protein
  
• Albumin
  
• Creatinine
  
• Trypsinogen
  
• pH
  
• Specific gravity

When the dipstick is dipped in urine, the reagents in each pad dissolve, initiating a chemical reaction that results in a colour change. After a set period of time (usually in the range 20–60 seconds) the colour change of each pad is compared with a colour chart provided by the manufacturer, and the result read. In the case of some urine constituents the test is semi-quantitative, in that the more intense the colour change, the higher is the concentration of the particular urine constituent. If a semi-quantitative result is possible, results are typically reported as either trace (lowest detectable concentration) or 1+, 2+, 3+ or 4+ (very high concentration). If a semi-quantitative result is not possible then the report is either positive (the substance is present in urine) or negative (the substance is not present in urine).

Each manufacturer of test strips provides detail of the test procedure that must be followed for accurate results, but some general points are made here:

• The chemical reagents contained on the strip pads have a limited shelf life ­governed by the expiry date on the container. Always check expiry date before use. Dipsticks should not be used if the expiry date has passed.
  
• The stability of the chemical reagents depend on the desiccant (which absorbs any moisture) provided either in a sachet or in the lid of the container. To maintain chemical stability it is important that dipsticks are stored in the container provided and that the lid is replaced immediately after use. Only remove enough dipsticks for immediate use.
  
• The chemicals on dipstick pads dissolve if left in urine long enough. The dipstick must be dipped in urine so that all reagent pads are immersed and then immediately removed.
  
• It is important that the chemicals from one pad do not contaminate the next. To avoid this, excess urine must be removed as the strip is withdrawn from the urine sample, by dragging the edge of the strip against the urine container and then immediately blotting the side of the strip on an absorbent paper towel.
  
• Good lighting is essential for accurate reading of colour change.
  
• Ability to accurately differentiate sometimes subtle colour change is essential. It may be inappropriate for staff with colour blindness to perform dipstick testing.
  
• The timing of the reaction is vital for accurate results; always observe manufacturers’ timing instructions.
  
• Urine is a biological fluid which must be treated as a potential source of ­pathogens, with risk to patient and staff of cross-infection. All staff performing dipstick testing must be familiar with local health and safety policy regarding work with body fluids (e.g. the need for protective gloves and apron, what to do in the event of a urine spillage etc.).
  
• Results must be recorded on a worksheet and in patients’ notes.
  
• The quality of results using dipsticks should be regularly assessed by a local laboratory quality control scheme³.

Interpretation of results

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