Dr. YK Amdekar writes on urine output, and how it is an important diagnostic marker for renal health and subsequently other major underlying diseases. In this article, he touches upon oliguria, practice points for diagnosis and the treatment approach to take.

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Introduction

Urine output is an important parameter that is easy to monitor and offers useful bedside information. Patients can also keep a note of it and therefore it forms a part of personal history that every doctor must enquire. A reduced amount of urine – oliguria – is a common occurrence in disease states but an excessive amount of urine – polyuria may also be a manifestation of a disease process. Urine output less than 400-500 ml per 24 hours in adults is considered oliguria. In infants, it is less than 1 ml/kg body weight/hour and in children less than 0.5 ml/kg/hour. Urine output of more than 2.5 litres per day in adults is considered polyuria, and it is more than 5 ml/kg/hour in infants and 4 ml/kg/hour in children.

Basics revisited

Glomerular filtration is the first step to make urine through which the excess water and waste products from the blood are excreted out of the body. Kidneys are supplied with 20% of cardiac output that amounts to more than one litre per minute in an adult. Afferent arterioles deliver blood to a glomerulus for filtration while efferent arterioles carry the filtrate into the excretory system and venules carry the blood back into the circulation.

Renal autoregulation can dilate or constrict afferent arterioles which counteract changes in blood pressure within limits. The rate at which kidneys filter blood is called the glomerular filtration rate (GFR). Serum creatinine increases only after GFR is reduced considerably to less than 30 ml/min from normal 100-120 ml/min and hence is a late indicator of impaired renal function.

Common causes of oliguria

Reduction in blood supply to kidneys, impairment of glomerular function and obstruction to urine outflow are the main groups of mechanisms that produce oliguria. Poor perfusion of kidneys may result from dehydration due to various causes (severe diarrhoea or vomiting, severe burns), capillary leak (dengue shock syndrome), poor cardiac output, systemic hypertension or renal artery disease (artery stenosis or arteritis). Glomerular dysfunction may be caused by glomerulonephritis (endothelial and interstitial diseases) and renal failure of different aetiology. Obstruction to urine outflow presents with oliguria in spite of normal renal function as in the case of bilateral pelvic-ureteric junction defect or posterior urethral valve. Neurological disorders may also lead to retained urine.

Clinical approach to oliguria

The first step is to confirm reduced urine output. It may be obvious or rarely may need to measure urine quantity, especially in the ICU setting. Poor intake of fluids in a sick individual may also have oliguria but it is not severe and also not a presenting feature. Enlarged bladder denotes either mechanical obstruction or neurological cause of poor urine output in spite of normal urine formation.

Once the obstruction is ruled out, one must look for signs of dehydration (tachycardia, dry mucosa, loss of skin turgor) or shock (cold extremities, hypotension, increased capillary refill time and encephalopathy) or cardiac failure (pedal oedema, engorged neck veins, cardiomegaly).

Hematuria (high coloured urine), facial puffiness, systemic hypertension suggests endothelial glomerular disease such as acute glomerulonephritis due to different causes or signs of renal failure (encephalopathy with deep rapid breaths suggestive of metabolic acidosis) of various aetiology.

Investigations

GFR can be estimated roughly, referred to as eGFR by following a simple equation. (there are other complicated methods of calculation).

Management

It would depend on the final diagnosis. Not every patient with oliguria would need intravenous fluids. It would be necessary only in case of dehydration or intravascular constriction. It may be harmful in case of cardiac failure and useless in case of an obstructed urinary system. Similarly, diuretic drugs are not required for every oliguric patient. Other details are out of the scope of this article.

Summary

Oliguria reduced urine output may be due to insufficient formation of urine or due to obstruction to the outlet of urine in spite of adequate production. If it is due to the reduced formation, one needs to differentiate between prerenal condition from renal pathology. It can be assessed easily on the basis of history and physical examination before embarking on investigations.