Investigations for urinary tract disease

Overview

Urological diagnosis rests on a small set of tests. Each one answers a particular question: Is there infection? Is there bleeding, and where from? Is there a stone? Is the kidney obstructed? Is the bladder outlet blocked?

Exam questions usually give a clinical scenario and ask for the single best next test. Learn the "question → test" pairs in this chapter.

Urine tests

Dipstick

A reagent strip checks glucose, ketones, bilirubin, urobilinogen, specific gravity, pH, protein, blood, nitrites and leucocyte esterase.

• Nitrites: most Gram-negative bacteria turn nitrate into nitrite. False negatives are common.
• Leucocyte esterase: marks pus cells.
• Blood: the pad reacts to haem, so haemoglobin and myoglobin also give a positive result without red cells.
• A trace of blood on dipstick should be treated as negative.

Mid-stream urine (MSU) microscopy and culture

The MSU confirms a UTI, names the organism and gives antibiotic sensitivities. Normal urine holds a few white cells (up to about 40 × 10⁶/L) and epithelial cells.

• Pyuria with no growth (sterile pyuria): think of TB, partly treated infection, stones, carcinoma in situ (CIS), chlamydia or gonorrhoea, and papillary necrosis.
• Red cell casts and dysmorphic red cells: point to a glomerular cause, so the patient needs a nephrology work-up.

Early-morning urine (EMU)

For suspected genitourinary TB, send three EMU samples on consecutive days for Ziehl–Neelsen staining and TB culture. Culture takes about 6 weeks.

Urine cytology

Looks for malignant urothelial cells.

• About 15% of low-grade transitional cell carcinomas give positive cytology, compared with about 50% of high-grade ones.
• A positive result is highly specific. It is most useful for CIS and high-grade disease, especially when cystoscopy looks normal.

Blood tests

• Serum creatinine stays normal with disease in one kidney if the other kidney works well.
• Cisplatin-based chemotherapy generally needs an eGFR of 50 mL/min or more.
• Kidney and bladder cancers have no routine serum marker.

Prostate-specific antigen (PSA)

PSA is a glycoprotein made by prostatic epithelium. Its normal job is to liquefy semen. It leaks into the blood when prostatic architecture is disturbed by BPH, prostatitis or cancer, so it is organ-specific but not cancer-specific.

Age-specific upper limits often quoted: 2.5 ng/mL (40–49 years), 3.5 (50–59), 4.5 (60–69) and 6.5 (70–79).

• A lower percentage of unbound (fPSA) PSA favours cancer.
• PSA velocity and density have added little in practice; PSA doubling time helps in men under surveillance.
• A bone scan is rarely positive until PSA is above about 40 ng/mL.

Endoscopy

Cystoscopy

• Flexible cystoscopy: done under local anaesthetic as an outpatient. Mainly diagnostic; small biopsies and stent removal are possible.
• Rigid cystoscopy: under general anaesthetic; allows resection, larger biopsies and ureteric instrumentation.
• Detection of flat lesions and CIS is improved by narrow band imaging and photodynamic (blue light) diagnosis, where a fluorescent agent instilled in the bladder makes tumour glow pink.

Retrograde studies and ureteroscopy

• Retrograde ureteropyelography: contrast is injected up a fine ureteric catheter placed at cystoscopy, outlining the ureter and pelvis (stones, tumours, strictures).
• Ureteroscopy: semi-rigid for the ureter; flexible for the renal pelvis and calyces. It is both diagnostic and therapeutic (stones, tumours).

Imaging

Ultrasound (USS)

• First-line, cheap, no radiation, can be done at the bedside.
• Excellent for hydronephrosis, renal masses and cysts, scarring, bladder tumours, bladder stones, a thick trabeculated bladder, diverticula and post-void residual.
• Stones cast an acoustic shadow, but USS misses some stones.
• Poor for the ureter unless it is dilated.
• Guides percutaneous nephrostomy insertion and is the key test for scrotal swellings.
• Transrectal ultrasound (TRUS) shows prostate zones and volume and guides biopsy. The classic cancer appearance is a hypoechoic area in the peripheral zone.

Plain KUB X-ray

Covers kidneys, ureters and bladder. Used to see whether a stone is radio-opaque, to follow stones during and after lithotripsy, and to check double-J stent position.

Intravenous urography (IVU)

Contrast is excreted by the kidneys and outlines the collecting system. It is now rarely used; its role in stones has been taken over by non-contrast CT. Classic IVU signs still appear in MCQs (e.g. cobra-head ureterocele, clubbed calyces, filling defects).

Computed tomography (CT)

The Bosniak classification of renal cysts (CT or MRI)

Magnetic resonance imaging (MRI)

• Stages several urological cancers.
• Multiparametric MRI of the prostate is now central before biopsy. Lesions are scored 1 to 5 on the PI-RADS scale and can be targeted.

Nuclear medicine

A normal MAG3 curve rises steeply, peaks at about 2–5 minutes, then falls as tracer drains. In obstruction the tracer stays in the pelvis even after a diuretic.

Whitaker test

A rarely needed pressure-flow test: fluid is run into the renal pelvis through a nephrostomy at 10 mL/min while pressure is measured. An abnormal rise in pressure confirms obstruction.

Lower tract contrast studies

• Micturating cystourethrogram (MCU): vesicoureteric reflux and posterior urethral valves.
• Retrograde (ascending) urethrogram: urethral stricture and urethral injury; combined with a descending study to plan urethroplasty.
• Cystogram: bladder rupture after trauma.

Tests of bladder function

Flow rate and residual

The first bladder tests for men with LUTS and women with recurrent UTI or LUTS. The man should void at least 150–200 mL, ideally on more than one occasion.

• A long, flat, plateau-shaped curve suggests a urethral stricture.
• Post-void residual is measured by ultrasound. A single high reading should be repeated.

Urodynamics (pressure-flow study)

Fine catheters in the bladder and rectum measure bladder and abdominal pressure, so true detrusor pressure is obtained by subtraction.

A normal bladder fills to about 400–550 mL with a pressure rise of less than 15 cmH₂O. Normal male voiding pressure is up to about 60 cmH₂O; above 80 cmH₂O is high.

Choosing the right test

Quick revision

• Trace blood on dipstick = negative; haem pad also detects myoglobin and haemoglobin.
• Sterile pyuria → think TB → three early-morning urines.
• Cytology: specific, insensitive; good for CIS and high grade.
• PSA: raised by UTI (6 weeks to settle), halved by 5α-reductase inhibitors, little changed by DRE.
• Stones: non-contrast CT; IVU obsolete; phleboliths mimic stones on KUB.
• DMSA = scars and split function; MAG3 + furosemide = obstruction.
• Bosniak III ~50% and IV ~100% malignant.
• Qmax above 15 reassuring, below 10 obstructed; plateau curve = stricture.
• Urodynamics separates obstruction from a weak detrusor.

Written by The House Job from standard references (Bailey & Love; Kumar and Clark's Clinical Medicine).

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