An unspun sample of urine may be examined by placing a drop on a slide using a pipettte, covering with a coverslip and examining by low-power and higher power microscopy. Phase-contrast microscopy is a helpful additional tool. Frequently, a spun-urine sample is examined. Urine is centrifuged, the supernatant is discarded and an aliquot of the residuum placed on a glass slide employing a Pasteur pipette.
Urine microscopy should be carried out in all patients suspected of having renal disease. Care must be taken to obtain a ‘clean’ sample of mid-stream urine . The presence of numerous skin squames suggests a contaminated, poorly collected sample that cannot be properly interpreted.
If a clean sample of urine cannot be obtained, suprapubic aspiration is required in suspected urinary-tract infections.
Most urines are examined by microscopy in hospital practice by microbiology laboratory technicians who must process large numbers of such urines each day. Best results are obtained in nephrological practice if microscopy is carried out by the physician caring for the patient.
The presence of 10 or more white blood cells (WBC) per cubic millimetre in fresh unspun mid-stream urine samples is abnormal and indicates an inflammatory reaction within the urinary tract. Most commonly it is due to urinary tract infection (UTI) but it may also be found in sterile urine in patients during antibiotic treatment of urinary infection or within 14 days of treatment. Sterile pyuria also occurs in patients with stones, tubulo-interstitial nephritis, papillary necrosis, tuberculosis, and interstitial cystitis.
The presence of one or more red cells per cubic millimetre in unspun urine samples results in a positive Stix test for blood and is abnormal. It is claimed that red cells of glomerular ongm can be identified by their dysmorphic appearance, especially on phase-contrast microscopy, but this is not yet generally available or accepted.
These cylindrical bodies, which are moulded (‘cast’) in the shape of the distal tubular lumen, may be hyaline, granular or cellular. Hyaline casts and fine granular casts represent precipitated protein and may be seen in normal urine, particularly after exercise. More coarsely granular casts occur with pathological proteinuria in glomerular and tubular disease. Red-cell casts-even one-always indicate renal disease. If red cells degenerate, a rustycoloured ‘haemoglobin’ granular cast is seen. White cell casts may be seen in acute pyelonephritis. They may be confused with the tubular cell casts that occur in patients with acute tubular necrosis.
The demonstration of bacteria on Gram staining of the centrifuged deposit of a clean-catch mid-stream urine sample is highly suggestive of urinary infection and can be of value in the immediate differential diagnosis of UTI. If accompanied by pyuria it may be accepted as evidence of UTI in the ill and febrile patient and treatment should be initiated.
Urine for quantitative culture must always be obtained prior to starting antibiotic treatment in order to confirm the diagnosis and to allow definition of bacterial antibiotic sensitivities.
Stix testing for blood or protein is of no value in the diagnosis of UTI, as both are absent from the urine of many patients with bacteriuria.
QUANTITATIVE TESTS OF RENAL FUNCTION
The use of blood urea, serum creatinine and GFR as measures of renal function is discussed. Quantir fication of proteinuri , including the investigation of selective proteinuria, is discussed on p. 455. Other quantitative tests of disturbed renal function such as measurements of urine output of calcium, sodium or potassium or urine acidification are described under the relevant disorders.
Plain X-ray A plain radiograph of the abdomen is always taken prior to urography. Its main value is to identify renal calcification or radio dense calculi in the kidney pelvis, line of the ureters or bladder (Fig. 9.6). Care must be taken in viewing the X-ray in order not to miss calculi obscured by bowel shadows or bone. Renal size and outline are best assessed during excretion urography or by ultrasound.
This is also known as intravenous urography (IVU) or intravenous pyelography (IvP). If carefully executed and properly interpreted, the urogram is one of the most valuable diagnostic tools for the investigation of renal disease. Carefully timed serial X-rays are taken of the kidneys and the full length of abdomen following a slow intravenous injection of an organic iodine-containing contrast medium. Films taken at the end of the injection show opacification of the parenchyma, allowing definition of size and renal outline. The kidneys are normally smooth in outline. In adults they measure 11-14 cm in length, differing by less than 2 cm. An irregular outline due to cortical scarring is abnormal. Reduction in size indicates chronic disease either primarily of the renal parenchyma or of the renal vasculature.
The application of a compression band to the abdomen, designed to partially obstruct ureteral emptying, helps distension of the upper tracts. Special attention is paid to the size, shape and disposition of the calyces and pelvis for evidence of anatomical abnormality such as calyceal clubbing, abnormal dilatation, cavitation or filling defects. The significance of these are described under the particular diseases.
After 10-20 min, the compression bands are removed and full-length films are obtained before and after voiding to study emptying of the upper tract, ureters and bladder. Reactions to the contrast media include anaphylactic reactions and, rarely, convulsions. Non-ionic contrast media have reduced these complications. Patients with a history of allergy to iodine, those who have had a previous contrast reaction, those with multiple allergies, and asthmatics should receive prednisolone 40 mg 24 hours before and on the day of the examination. Contrast media may also be nephrotoxic