Polyuria

In both polyuria and high urinary frequency the patient will be passing urine more often than before. But in polyuria, patients will pass abnormally large volumes of clear urine each time. In urinary frequency, the volume voided each time will be normal or reduced. The only way of knowing objectively whether this is the case is by collecting a 24-hour urine sample (>3 L = polyuria). As this is usually impractical outside of hospital, one must rely on the patient’s recall, with the caveat that many patients find it difficult to estimate urine output. In the history, then, ask them whether they feel they are passing a larger volume every time they go to the toilet. Remember that it is important to be as sure as you can that you are dealing with polyuria and not urinary frequency or nocturia, as the differential diagnoses are quite distinct. Chronic renal failure and hypercalcaemia (e.g. due to bone metastases) cause polyuria by inducing nephrogenic diabetes insipidus. Similarly, steroids and Cushing’s syndrome can cause polyuria by causing diabetes mellitus. • Temporal pattern of urine output (number of times in the day and at night), especially nocturia. At night, the kidneys concentrate urine in order to retain fluid (as intake is zero), removing the need to urinate during sleep. Thus nocturia (in the absence of other causes of nocturia, e.g. benign prostatic hyperplasia (BPH)) is often one of the earliest signs of a loss of concentrating ability. This symptom makes primary polydipsia less likely. • Fatigue, weight loss, recurrent infections. All can be features of diabetes mellitus. • Lower urinary tract symptoms (LUTS), e.g. frequency, urgency, hesitancy, terminal dribbling, incomplete voiding. These symptoms indicate pathology in the bladder or the outflow tract, e.g. prostatism in men, detrusor instability and prolapse in women. Not strictly speaking polyuria. • Pain, frequency, change in urine colour and smell. These are all symptoms suggestive of a urinary tract infection (UTI), which would cause increased frequency but not polyuria. • Past medical history. Look for any history of renal problems or conditions that may precipitate chronic renal failure (e.g. vasculitides, hypertension, chronic urinary retention).