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In a population of 432 apparently healthy men aged 50 years and over 19-2% had one or more erythrocytes per high-power field in the urine while 8-1% had more than 10. Erythrocyturia seemed to be strongly correlated with tobacco consumption. No significant association with occupation could be demonstrated but the data were insufficient to refute the possibility of such an association. Raised levels of orthoaminophenols as a result of abnormal tryptophan metabolism induced by smoking might cause microhaematuria in smokers. As these metabolites are carcinogens microhaematuria might be a valuable sign of cancer of the urinary bladder.

During a survey for bladder cancer in a healthy male population a surprisingly high incidence of significant microhaematuria was found and no urinary sample was completely free of blood. A study of the sensitivity of the different techniques for assessing microhaematuria seemed necessary. A semiquantitative centrifugation technique, developed for cytological purposes, was found to be the most sensitive. Blood could be detected in concentrations of less than 50 RBC/ml urine. With conventional analysis, performed carefully, the minimum detectable concentration was about 500 RBC/ml. Reagent strips, based on the orthotholidin peroxidase reaction and developed specifically for urine analysis, gave positive reactions in minimum concentrations of 1 000 000 RBC/ml (original Hemastix), 50 000 RBC/ml (new Hemastix), and 20 000 RBC/ml (Sangur-Test). Positive scores with lower concentrations could be obtained with the Sangur-Test strip when it was read with a magnifying glass. However, the degree of erythrocyturia that may be regarded as physiological and that which is pathological has yet to be defined.