After finding that American Urological Association (AUA) practice recommendations for microhematuria follow-up did not perform well in identifying which patients were most likely to have malignant tumors of the urinary tract, urologist Ronald K. Loo, MD, of Southern California Permanente Medical Group (Downey, Calif.), and colleagues created and validated a Hematuria Risk Index.

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What were the most significant findings from your study, “Stratifying risk of urinary tract malignant tumors in patients with asymptomatic microscopic hematuria” (Mayo Clinic Proceedings 2013;88[2]:129-138)?

Dr. Loo: Microhematuria alone is a very poor indicator of urinary tract malignancy. Overall, it is a relatively poor indicator for bladder cancer, and likely has no correlation whatsoever with renal cancer.

However, when combined with other common parameters (age, gender, smoking history, history of gross hematuria), it may be possible to better predict who will and who will not benefit from urologic evaluation and imaging.

How prevalent is asymptomatic microscopic hematuria (AMH)?

Dr. Loo: AMH in the general population is common. The prevalence of some degree of hematuria has been reported to be as high as 9% to 18% in large screening studies of apparently normal individuals. Dr. Albert Mariani [a co-creator of the Hematuria Risk Index] confirmed these findings in 1,346 healthy asymptomatic patients ages 18 to 91, and measured some degree of hematuria by urinalysis in 17.4% (The Journal of Urology 1984;132[1]:64-66).

What are the most likely causes of AMH? Do these differ from the most likely causes of symptomatic microscopic hematuria?

Dr. Loo: The most common reasons for asymptomatic microscopic hematuria are clinically insignificant, without urologic or nephrologic etiology, because microscopic hematuria occurs so frequently in the normal population. 

The most common benign causes of AMH include strenuous exercise (runner’s hematuria), menstruation, and subclinical urinary tract infection (UTI). The most common urologic causes of AMH include urinary calculi, prostatic bleeding, and infection.

The most likely causes of symptomatic microscopic hematuria will also include infection (cystitis, prostatitis, urethritis) and urinary calculi (renal, ureteral, bladder).  Trauma and obstruction from papillary necrosis also are possibilities, but a thorough history should provide clues such as chronic anticoagulation and flank trauma, diabetes, or sickle cell.

Nephrologic causes of microscopic hematuria must also be considered. Patients may or may not be symptomatic, but will generally exhibit other signs such as hypertension, renal insufficiency, proteinuria, or urinary casts and/or dysmorphic red blood cells on microscopic urinalysis.

How likely is a person with microscopic hematuria to have cancer of the urinary tract, and how likely is that tumor to be malignant?

Dr. Loo: Any asymptomatic person with microscopic hematuria is unlikely to have a urinary cancer. The AUA’s pooled urinary tract malignancy rate was 3.3%, and in our study it was 2.5%. Another important factor to consider is that only 0.3% of our patients were found to have a renal cancer.

We’re not sure if microscopic hematuria correlates at all with renal cancer, as Dr. [J. Stuart] Wolf previously reported on a series of more than 6,000 asymptomatic individuals ages 50 to 79 with a confirmed renal cancer rate of only 0.33% (The Journal of Urology 1998;159[4]:1120-1133). So, patients with microscopic hematuria have about a 3% risk of having a urinary malignancy, and it is most likely to be a bladder cancer.

However, according to our study, there may be more predictive ways to determine who is more likely and less likely to have a malignancy using common parameters, which include age, gender, smoking history, and a history of gross hematuria.

Is asymptomatic microscopic hematuria more or less likely than asymptomatic gross hematuria to indicate cancer?

Dr. Loo: Any patient with a history of gross hematuria is about seven times more likely to have a urinary malignancy than a patient with AMH only.

Our findings appear to correlate well with Dr. Mariani’s report of 1,000 patients, in which the risk of finding a genitourinary tract malignant tumor was 6.36 times higher in patients with a history of gross hematuria (The Journal of Urology 1989;141[2]:350-355).