Best Practice Points, Urine Dipstick

It is important to note that there is no correlation between the extent of haematuria in a patient and the extent and severity of underlying disease. In addition, up to 4% of patients with normal dipsticks have abnormal microscopy [3]. In the standard urine dipstick test, leucocyte esterase detects the presence of white blood cells present in the urine as a consequence of inflammation or infection. Remember, a negative dipstick result does not exclude a UTI. False positive readings are far less common than false negative results.

Other causes of sterile pyuria:

• Stones
• Tubulointerstial nephritis
• Papillary necrosis
• TB
• Interstitial nephritis

White cells can persist in urine for some time after treatment so, again, it is important that symptoms are sought and treatment not started blindly.

Learning bite

A negative urine dipstick does not rule out urinary tract infection.

Nitrates

Nitrates are converted to nitrites in the bladder following a period of incubation with organisms. This is most reliable when used on the first void of the day. Nitrite detection has a positive predictive value of 96% for urinary tract infection, but a low negative predictive value. Acid pH will rule out renal tubular acidosis as a cause of nephrolithiasis, and alkaline pH typically results from infection with organisms that split urea with urease e.g. proteus and pseudomonas.

Urine culture

Routine urine culture is not needed in a young female with typical symptoms and positive nitrites and leucocytes. In all other cases – especially males, the immunocompromised, pregnant women and those with UGT obstruction and treatment failures – it is essential. Males have a longer urethra; UTIs are therefore less common and may indicate underlying pathology. Recurrent persistent infection needs investigation to exclude sinister causes.

Learning bite

Delays in transporting samples to the lab, especially leaving them in the warm environment of the emergency department, produces false positive results. Therefore, samples should be transported and refrigerated promptly.

Dipstick blood test

The dipstick test for blood looks at haemoglobin-like components and does not differentiate between haemoglobin from red cells and myoglobin from the breakdown of skeletal muscle. Note, also, that there is no correlation between the quantity of red blood cells seen and the degree of obstruction. Likewise, total obstruction may initially have no red cells present.

Study

Sugimura et al studied 823 patients presenting with microhaematuria over 5 years, between 1993 and 1998. They established the respective positive predictive values for a positive dipstick in bladder cancer of 1.7% and renal cancer of 0.2%. In patients over 50 years old the risk of bladder cancer in those with microscopic haematuria rose to 6.2% [12]. Long term follow up studies show those with a negative microscopy have no increase in risk of urological disease [3].

A negative repeat dipstick should not cease further investigation, as it has been found that patients with an initial positive test may still have an underlying abnormality, even if subsequent repeated tests are normal.