Hyperuricemia overproducers

Although rarely performed, a 24-hour urine collection can be obtained to determine if the patient is an overproducer or an underexcretor of uric acid. Individuals who excrete more than 800 mg of uric acid in this collection are considered overproducers. Patients with hyperuricemia who excrete less than 600 mg/day are classified as underexcretors of uric acid. 

Several enzyme systems regulate purine metabolism. Abnormalities in these regulatory systems can result in overproduction of uric acid. Uric acid also may be overproduced as a consequence of increased breakdown of tissue nucleic acids, as with myeloproliferative and lymphoproliferative disorders. Dietary purines play an unimportant role in the generation of hyperuricemia in the absence of some derangement in purine metabolism or elimination. 

The pathophysiologic approach to the evaluation of hyperuricemia requires determining whether the patient is overproducing or underexcreting uric acid. This can be accomplished by placing the patient on a purine-free diet for 3 to 5 days and then measuring the amount of uric acid excreted in the urine in 24 hours. Normal individuals produce 600 to 800 mg of uric acid daily and excrete less than 600 mg in urine. Individuals who excrete more than 600 mg on a purine-free diet may be considered overproducers. Hyperuricemic individuals who excrete less than 600 mg of uric acid per 24 hours... 

Secondary (or acquired) gout is caused by seemingly unrelated disorders. These conditions may cause hyperuricemia by either overproduction of uric acid or its undersecretion by the kidneys. For example, leukemia patients overproduce uric acid either because of massive cell destruction or the chemotherapy treatment required to destroy the cancerous cells. Hyperuricemia also results when certain drugs interfere with the renal secretion of uric acid into the urine. Patients with lead poisoning are also likely to develop gout because of renal damage. 

Isolated hyperuricemia is not necessarily an indication for therapy, as not all of these patients develop gout. Persistently elevated uric acid levels, complicated by recurrent gouty arthritis, nephropathy, or subcutaneous tophi, can be lowered by allopurinol, which inhibits the formation of urate, or by uricosuric agents. Some physicians recommend measuring 24-hour urinary urate levels in patients who are on a low-purine diet to distinguish underexcretors from overproducers. However, tailored and empirical therapies have similar outcomes. 

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