From the Guidelines
Laboratory findings suggestive of urate nephropathy, as opposed to other forms of chronic kidney disease, include hyperuricemia (serum uric acid levels >7 mg/dL in men or >6 mg/dL in women), acidic urine pH (typically <5.5), and the presence of uric acid crystals in urine sediment. Patients may also show elevated blood urea nitrogen (BUN) and creatinine levels indicating reduced kidney function, as supported by a retrospective observational study of U.S. veterans with gout, which found a 2-fold difference in new diagnoses of kidney disease in patients with high versus low serum urate levels 1. Urine analysis often reveals low urinary volume and low urinary pH, creating conditions favorable for uric acid crystal formation. Imaging studies might show radiolucent uric acid stones or nephrocalcinosis. In contrast to other forms of CKD, urate nephropathy patients typically have a history of gout, excessive purine intake, or conditions with high cell turnover like leukemia or tumor lysis syndrome. The absence of other markers such as significant proteinuria, hematuria, or specific autoantibodies helps distinguish urate nephropathy from glomerular diseases. Additionally, a therapeutic response to urate-lowering therapy such as allopurinol (starting at 100mg daily, gradually increasing to 300-600mg daily as needed) or febuxostat (40-80mg daily) with improvement in kidney function supports the diagnosis of urate nephropathy, as achieving a serum urate level less than 357 µmol/L (<6.0 mg/dL) at the end of 1 year has been associated with a reduced risk for acute gout attacks of approximately 5% 1. Key laboratory findings to differentiate urate nephropathy from other forms of CKD include:
- Hyperuricemia
- Acidic urine pH
- Presence of uric acid crystals in urine sediment
- Elevated BUN and creatinine levels
- Low urinary volume and pH
- Radiolucent uric acid stones or nephrocalcinosis on imaging studies. It is essential to monitor serum urate levels to assess the effectiveness of urate-lowering therapy, as supported by a post hoc analysis that combined data from 3 large trials, which found that achieved urate levels were associated with acute gout attacks requiring treatment 1.
From the Research
Lab Findings Suggestive of Urate Nephropathy
- Elevated serum urate levels, particularly in the context of increased cell lysis, can lead to intrarenal precipitation and acute uric acid nephropathy 2
- A urinary uric acid-to-creatinine ratio greater than 1 can help distinguish acute uric acid nephropathy from other forms of acute renal failure 2, 3
- The presence of urate crystals in the urine sediment can be a guide for the treatment of asymptomatic hyperuricemia and may indicate urate nephropathy 4
Differentiation from Other Forms of Chronic Kidney Disease
- Serum uric acid levels have been linked to the development and progression of chronic kidney disease (CKD), but the relationship is complex and influenced by various factors 5
- Uric acid-lowering therapies may prevent or delay the progression of CKD, but the treatment of asymptomatic hyperuricemia is not recommended nor excluded by current guidelines 4, 5
- The ratio of uric acid to creatinine concentration in a random urine specimen can differentiate between acute uric acid nephropathy and other types of acute renal failure, but may not be accurate for estimating uric acid overexcretion in patients with gout 3, 6
Diagnostic Considerations
- A spot urine uric acid to creatinine ratio may not be an accurate indicator of uric acid overexcretion in patients with gout, and 24-hour urinary uric acid excretion may be a more reliable measure 6
- Trends in creatinine, proteinuria, and serum urate levels, as well as musculoskeletal ultrasound, may be useful in guiding the treatment of asymptomatic hyperuricemia and monitoring for urate nephropathy 4