Urine tests - Investigations and procedures

Investigations and procedures

Urine tests

Dipstick testing

The basic test includes blood, protein, glucose, specific gravity and urinary pH. More complex dipstick tests are available which test for ketones (raised in starvation and diabetic ketoacidosis), leucocyte esterase, which signifies white cells, and nitrites, which signifies the presence of certain bacteria that convert nitrates (found in normal urine) to nitrites.

·        Blood: Dipstick testing for blood is much more sensitive than the naked eye. False positives occur with myoglobinuria and haemoglobinuria. Cross-contamination may occur if females are menstruating. If truly positive, it should be investigated as for haematuria.

·        Protein: Normal protein excretion is <150mg/day, of which albumin normally forms about 35 mg. Dipstick for protein will be negative unless protein excretion is >300–500 mg/day. It mainly detects urinary albumin, and is insensitive for other proteins such as immunoglobulin light chains (Bence Jones protein) which require specific tests. Very dilute urine will underestimate the degree of proteinuria, and concentrated normal urine may show a trace of protein. If 1+ or more of protein is detected, further quantification of protein should be done (see below). False positives can occur with radiocontrast agents.

·        Glucose: Presence of glucose is almost always due to diabetes mellitus. Glucose is not normally found in the urine until the plasma glucose concentration is â‰¥10 mmol/L. If found with lower levels, this suggests an inability of the kidney to reabsorb filtered glucose due to dysfunction in the proximal tubule, such as occurs in multiple myeloma, renal tubular acidosis (RTA) and pregnancy.

·        Specific gravity (SG): This gives an estimation of urine concentration, it is useful in acute renal failure, investigation of polyuria or syndrome of inappropriate anti-diuretic hormone (SIADH). If there are high levels of glucose or myeloma chains in the urine, SG is high even when the patient is not dehydrated.

·        Urinary pH: In metabolic acidosis, urine pH falls to below 5.3, unless the cause of metabolic acidosis is RTA. Urinary tract infections with bacteria such as Proteus, which produce urease, cause the urine pH to rise to neutral or even alkaline levels.

Quantification of proteinuria

This is done on patients who have persistent proteinuria. There are two main methods.

1. 24-hour urine collection: Unfortunately this method is inaccurate and often inconvenient for the patient. They must empty their bladder when they get up in the morning, then begin collecting in a container every single drop of urine passed for the next 24 hours, up to and including the first emptying of the bladder the next morning. This is then sent to the laboratory for total protein quantification.

2. Protein-to-creatinine ratio: This is performed on a random urine specimen and is accurate, straightforward and convenient. This test is simpler using conventional units as used in the USA, because urinary protein concentration and serum creatinine concentration are both expressed in mg/dL, so a ratio of 3.8 represents a 24-hour protein excretion of 3.8 g/1.73 m2. In the United Kinddom, urinary creatinine is expressed in mmol/L so the result needs to be multiplied by 0.088:

Protein excretion (g/1.73m2) = Urinary protein (mg/dL) × 0.088 / Urine creatinine (mmol/L)

Microalbuminuria

Patients with proteinuria, which is greater than normal but less than detectable on dipstick, have ‘microalbuminuria’. This is defined as albumin excretion of between 35 and 200 mg/24 hour. It is an early indicator of diabetic kidney disease, and is also found in other conditions such as cardiovascular disease even without renal impairment. There are now bedside testing kits for detecting microal-buminuria (extra sensitive forms of dipsticks), but laboratory testing is more accurate, using a random urine sample to compare urinary albumin-to-creatinine concentration (>3 is abnormal).

Urine microscopy

This is performed on fresh mid-stream urine.

·        White blood cells: >10 WBCs per high power field (HPF) on uncentrifuged urine is abnormal. Causes include cystitis, tubulointerstitial nephritis and calculi.

·        Red blood cells: >1 RBC per high power field is abnormal. Red cells from the lower urinary tract look like normal, round RBCs seen on a blood film. If they are of variable size and shape, with blebs, budding or as though a ‘bite’ has been taken out of an edge, they are dysmorphic RBCs, which are a sign of glomerular disease.

·        Bacteria: Visible bacteria may be due to contamination of the specimen, or a urinary tract infection. Whether treatment is needed depends on the number of WBCs per HPF present.

·        Crystals: Uric acid crystals and calcium oxalate or calcium phosphate crystals are commonly seen in normal urine. Other types may signify an underlying disease.

·        Casts: These are cylinders formed in the renal tubules from Tamm–Horsfall protein, which is normally secreted by tubular epithelium. In glomerular or tubular disease, cells in the urine become incorporated into the casts. Red cell casts are diagnostic of glomerular disease. White cell casts occur in tubulointerstitial disease and pyelonephritis. Other sorts of casts such as granular or epithelial cell casts exist.

Urinary electrolytes

Comparing urinary and serum sodium concentration is useful in the assessment of fluid balance. In hypona-traemia, a low urinary sodium is physiological, whereas a high urinary sodium suggests renal failure. In a patient with a normal serum sodium, a low urinary sodium indicates salt-and-water depletion (dehydration). Urinary sodium is also useful in differentiating types of acute renal failure. Following abdominal or pelvic surgery, it can be useful to measure urea and creatinine concentrations in fluid from drains or aspirated from a collection. If these are similar to the urine urea and creatinine concentrations, this indicates a urinary leak.