a. Uric acid. Elevated uric acid values occur in about 90% of gout patients.
b. Creatinine. Elevated levels may reflect gout-related nephropathy or primary renal disease.
2. Urine studies. Patients with primary gout can be subdivided into over-excreters of uric acid (15% of patients) and normal excreters based on quantitative urinary excretion of urate. On a regular diet, normal persons will excrete 300 to 800 mg of uric acid daily. The patient should be instructed to avoid alcohol or aspirin ingestion and to eat no more than one moderate serving of meat daily during the 3 days preceding the urine collection. Foods with a very high purine content, such as organ meats (liver and kidney) and anchovies, should also be avoided before the collection. A uric acid collection obtained during an acute gout attack is unreliable as a result of fluctuating serum acid levels not usually seen in the basal state.
3. Radiographs. Soft-tissue swelling and osteopenia may occur as nonspecific early changes on radiographs. As the disease progresses, soft-tissue tophi may be seen as well as punched-out, sharply marginated areas of bony destruction. Pure urate stones are radiolucent, and imaging studies must take this into account.
C. The differential diagnosis includes RA, osteoarthritis, and pseudogout. At times, it can be exceedingly difficult to distinguish rheumatoid nodules from tophi without the aid of a biopsy specimen or aspirate. The absence of crystals in the synovial fluid distinguishes chronic gout from osteoarthritis. The demonstration of calcium pyrophosphate crystals in synovial fluid as well as chondrocalcinosis on radiography may help to distinguish chronic gouty arthritis from pseudogout.
D. Complications. Pure urate stones are found in approximately 80% to 90% of all gout patients in whom urolithiasis develops, in comparison with 10% of the general population. Any patient with gout in whom urolithiasis develops should have recovered stones analyzed chemically, and a 24-hour urine specimen should be obtained for determination of uric acid, calcium, and phosphate excretion. Other diseases associated with stone formation include hyperparathyroidism, cystinuria, and renal tubular acidosis, all of which require appropriate serum and urine studies to elucidate the etiology of urolithiasis. The treatment of choice in nephrolithiasis of gout is allopurinol, which lowers both serum and urinary uric acid values. Patients should be encouraged to maintain high urine volumes, and particularly in large "over-producers" of uric acid, alkalinization of the urine is warranted.
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