Amorphous Urate Crystals in Urinalysis
Amorphous urate crystals in a urinalysis are typically a benign, non-pathologic finding that indicates concentrated, acidic urine and do not, by themselves, signify disease or require treatment. 1
Clinical Significance
Amorphous urates are a normal finding that results from precipitation of uric acid salts in concentrated urine with acidic pH, and their presence does not indicate gout, kidney stones, or other uric acid-related pathology. 2 These crystals form when urine becomes supersaturated with urate in an acidic environment, but this is a physiologic phenomenon distinct from pathologic crystalluria. 3
Key Distinguishing Features from Pathologic Conditions:
Not diagnostic of gout: The diagnosis of gout requires identification of monosodium urate (MSU) crystals in synovial fluid or tophus aspirates—not urine crystals—and cannot be made based on hyperuricemia or urinary findings alone. 1
Not indicative of uric acid nephrolithiasis: Uric acid stone formation is indicated by radiolucent stones on imaging, persistent undue urine acidity, and uric acid crystals (not amorphous urates) in fresh urine samples. 4 The presence of amorphous urates does not predict stone formation. 1
Different from pathologic crystalluria: Pathologic crystalluria requires consideration of multiple criteria including crystal identity, abundance, aggregation patterns, occurrence on serial samples, and clinical context such as nephrolithiasis or renal failure—none of which apply to routine amorphous urates. 3
Practical Laboratory Considerations
The main clinical relevance of amorphous urates is that they can obscure other significant findings during microscopic urinalysis, such as bacteria, yeast, red blood cells, or white blood cells. 2
Management of Obscuring Crystals:
Prewarming the specimen to 60°C for 90 seconds before testing dissolves most amorphous urates and is the preferred protocol when crystals interfere with analysis. 2
Adding 50 mM sodium hydroxide can dissolve amorphous urates to enhance visibility of bacteria and yeast, but this has a deleterious effect on WBC and RBC counts and should be avoided when accurate cell counts are needed. 2
When to Consider Further Evaluation
While amorphous urates themselves are benign, certain clinical contexts warrant additional investigation:
Recurrent kidney stones: Perform stone analysis when available, and consider 24-hour urine collection for uric acid in patients with early-onset gout or renal stones. 1, 5
Suspected gout: Pursue synovial fluid aspiration and crystal analysis rather than relying on urinalysis findings. 1
Persistent acidic urine with stone history: Evaluate for uric acid nephrolithiasis with imaging and metabolic workup. 1, 4