- Author: Yasir Qazi, MD; Chief Editor: Vecihi Batuman, MD, FACP, FASN more...
Laboratory studies may include the following:
Serum uric acid
Complete blood cell count (CBC): Values may be abnormal in patients with hemolytic anemia, hematologic malignancies, or lead poisoning.
Electrolytes, BUN, and serum creatinine values: These are abnormal in patients with acidosis or renal disease.
Liver function tests: These are part of the general workup for patients with a possible malignancy or metabolic disorders; in addition, the results are useful as a baseline if allopurinol is used for treatment
Serum glucose level: This may be abnormal in patients with diabetes or glycogen storage diseases.
Lipid profile: Results are abnormal in those with dyslipidemia.
Calcium and phosphate levels: This measurement is needed for the workup of hyperparathyroidism, sarcoidosis, myeloma, and renal disease.
Thyroid-stimulating hormone level: Obtain this value to help rule out hypothyroidism.
Urinary uric acid excretion
Fractional excretion of urate on a low-purine diet
Spot urine ratio of uric acid to creatinine
Urinary uric acid secretion
If uric acid levels are found to be persistently elevated, an estimation of total uric acid excretion may be needed. The estimation of uric acid excretion is recommended in young males who are hyperuricemic, females who are premenopausal, people with a serum uric acid value greater than 11 mg/dL, and patients with gout.
One protocol recommends obtaining two 24-hour urine collections for creatinine clearance and uric acid excretion. The first collection is performed while patients are on their usual diet and alcohol intake. At the end of the first 24-hour collection, serum creatinine and urate levels are checked for an estimation of the creatinine clearance. The patient then goes on a low-purine, alcohol-free diet for 6 days, with a repeat 24-hour urine collection performed on the last day, followed by a serum creatinine and uric acid evaluation.
Depending on the 24-hour urine uric acid levels before the purine-restricted diet and after the purine-restricted diet, patients who are hyperuricemic can be categorized into the following three groups:
High-purine intake - Prediet value greater than 6 mmol/d, postdiet value less than 4 mmol/d
Overproducers - Prediet value greater than 6 mmol/d, postdiet value greater than 4.5 mmol/d
Underexcretors - Prediet value less than 6 mmol/d, postdiet value less than 2 mmol/d
Fractional excretion of urate on a low-purine diet
This test should be used to investigate the degree of underexcretion in patients with hyperuricemia or gout in patients for whom the cause cannot be determined. The fractional excretion of urate is calculated by the following formula:
Fractional excretion of urate = [(urine uric acid)×(serum creatinine)×(100%)]÷[(serum uric acid)×(urine creatinine)]
The reference intervals for patients on a low-purine diet and normal renal function are as follows:
Males - 7-9.5%
Females - 10-14%
Children - 15-22%
Values less than the lower limits of the reference range indicate underexcretion. The formula also circumvents any inaccuracy that may have occurred during urine collection.
Spot urine ratio of uric acid to creatinine
If a 24-hour urine collection is not possible, measure the ratio of uric acid to creatinine from a spot urine collection. A ratio greater than 0.8 indicates overproduction.
The ratio also helps differentiate acute uric acid nephropathy from the hyperuricemia that occurs secondary to renal failure. The ratio is greater than 0.9 in acute uric acid nephropathy and usually less than 0.7 in hyperuricemia secondary to renal insufficiency.
In patients with gout, radiographs may reveal evidence of joint swelling and subcortical cysts. In patients with hyperuricemia and renal disease, a renal sonogram is an important tool for kidney evaluation. Images from this study also may reveal the presence of uric acid stones.
Joint aspiration may be important in the diagnosis of acute gouty arthritis, in which uric acid crystals are found to be negatively birefringent under polarized microscopy.
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