🔬 Urinalysis

A rapid biochemical screen of urine using reagent strips to detect various substances including blood, protein, glucose, and markers of infection.

Indications include suspected urinary tract infection (dysuria, frequency), investigation of suspected renal disease (oedema, hypertension), or metabolic screening (suspected diabetes). It is used for the 'work-up' of many acute presentations, including abdominal pain, confusion in the elderly (delirium), and multisystem flares (e.g., SLE). It is also used in routine health checks and antenatal care.

A fresh 'mid-stream' urine (MSU) sample is collected in a clean container. A reagent strip (dipstick) is briefly immersed in the urine, ensuring all pads are covered, and then withdrawn. The strip is held horizontally to prevent cross-contamination between pads. After the specified time (stamped on the container), the colour change on each pad is compared against the reference scale. Proper timing is critical for accuracy.

Normal urine is typically clear and pale yellow. A normal dipstick result is negative for glucose, ketones, bilirubin, blood, nitrites, and leukocyte esterase. Protein should be negative or 'trace'. The pH usually ranges between 4.5 and 8.0, and Specific Gravity typically ranges between 1.005 and 1.030. Normal urobilinogen is present in small amounts.

Results are interpreted by comparing the colour change on the reagent pads to a standard chart provided by the manufacturer at specific timed intervals (e.g., 30-60 seconds). A positive nitrite test is highly suggestive of infection, but a negative test does not exclude it (as some organisms do not produce nitrites). Proteinuria should be followed up with a Protein-Creatinine Ratio (PCR) or Albumin-Creatinine Ratio (ACR). Hematuria always requires further investigation to rule out malignancy in appropriate age groups.

Leukocytes and Nitrites: Together suggest UTI (Nitrites are highly specific for Gram-negative bacteria like E. coli). Protein: Suggests renal disease, pre-eclampsia, or overflow proteinuria. Glucose/Ketones: Suggest diabetes mellitus or ketoacidosis. Blood: Hematuria may indicate infection, malignancy (urological), or calculi. Bilirubin/Urobilinogen: Suggest hepatobiliary disease or haemolysis. Specific Gravity: High in dehydration, low in diabetes insipidus.

Urinalysis is a rapid, point-of-care screening tool used extensively across all medical specialties in the UK. It is essential for the initial screen for Urinary Tract Infections (UTI), underlying renal disease (e.g., glomerulonephritis), and metabolic derangements like DKA. It is a mandatory screening test in every antenatal appointment to check for asymptomatic bacteriuria and proteinuria (as a marker for pre-eclampsia).

Common pitfalls include checking the pads at the wrong time (leading to false positives for glucose or leukocytes), using expired strips, or using a contaminated sample (e.g., vaginal discharge causing false-positive protein and leukocytes). Failing to hold the strip horizontally can cause 'run-over' of chemicals between pads, invalidating the result.

Dipsticks are screening tools, not definitive diagnostics. They can yield false positives; for example, blood can be positive due to myoglobinuria (rhabdomyolysis) or menstruation. Nitrites can be false-negative if the urine has not been in the bladder long enough for bacteria to convert nitrates. Dehydration can cause 'concentrated' urine, leading to trace readings of protein or blood that may not be clinically significant.

Students should know the 'positive' combinations that suggest UTI (Leukocytes + Nitrites). They must be aware that in patients >65 years, dipsticks are unreliable for UTI diagnosis due to asymptomatic bacteriuria; clinical symptoms are paramount. Non-visible haematuria management protocols (NICE) are also high-yield.