Anuria is de.ned as complete absence of urine production and usually indicates obstruction to the urinary tract. The level of obstruction may be at the outlet of the bladder, or at the level of the ureters bilaterally. Unrelieved bilateral urinary tract obstruction leads rapidly to acute renal failure, which may have very serious consequences (e.g., hyperkalaemia, .uid overload). If the level of obstruction is at the outlet of the bladder, abdominal examination will reveal a percussable and palpably distended bladder. Urine will be present in the bladder on catheterisation, and urine output will resume once a catheter has bypassed the obstruction. The commonest cause is benign prostatic enlargement and less commonly malignant enlargement of the prostate.

If the obstruction is at the level of the lower ureters or ureteric ori.ces, the bladder will not be palpable or percussable. Catheterisation will reveal no or a very low volume of urine in the bladder and there will be no improvement in urine output, or of renal function post-catheterisation. Causes include locally advanced prostate cancer, extensive involvement of the trigone of the bladder by bladder cancer, and locally advanced cervical or rectal cancer. Rectal or vaginal examination may reveal a cervical, prostatic, or rectal cancer and cystoscopic examination of the bladder may demonstrate a bladder cancer.

Bilateral obstruction higher up the ureters may be due to extensive lymph node metastases to the pelvic and para-aortic nodes from distant malignancy, retroperitoneal .brosis, and rarely bilateral ureteric stones. Evidence of a malignancy elsewhere may be found on clinical examination. The diagnosis is usually made on the basis of excluding obstruction at the outlet of the bladder and in the lower ureters and by radiographic imaging (ultrasound and abdominal CT).

Oliguria is scanty urine production, and more precisely is de.ned as urine production of less than 400 mL/day in adults and less than 1 mL/kg of bodyweight per hour in children. The causes are prerenal (e.g., hypovolaemia, hypotension), renal (e.g., acute vasculitis, acute glomerular lesions, acute interstitial nephritis, and acute tubular necrosis from nephrotoxic drugs, toxins, or sepsis), and postrenal causes (as for anuria, but where the degree of obstruction has not yet reached a level critical enough to stop urine production completely).