Urinalysis Findings Indicating Kidney Stones
Hematuria (blood in urine) is the primary urinalysis finding that indicates kidney stones, along with specific urine pH patterns and characteristic crystals on microscopic examination. 1
Key Urinalysis Indicators
Hematuria (Most Common Finding)
- Microscopic hematuria (≥3 RBCs/HPF) is present in the majority of patients with kidney stones and represents the most frequent urinalysis abnormality associated with nephrolithiasis 1, 2
- Gross (visible) hematuria can occur with kidney stones, particularly when stones cause trauma to the urinary tract during passage 2
- Dipstick testing showing blood (trace or greater) should always be confirmed with microscopic examination, as dipstick alone has limited specificity (65-99%) and can produce false positives from myoglobinuria or other substances 3
Urine pH Patterns
- Acidic urine pH (<5.5) strongly suggests uric acid stone formation, as unduly acidic urine is the most frequent biochemical diagnosis in uric acid stones 1, 4
- Alkaline urine pH (>7.0) suggests calcium phosphate or struvite (infection) stones 1, 5
- Urine pH assessment is critical because it provides information about stone-forming factors even when the actual stone cannot be retrieved 5
Crystal Identification on Microscopy
- Specific crystals visible on microscopic examination can be pathognomonic (definitively diagnostic) of stone type 1
- Cystine crystals are pathognomonic for cystine stones and should prompt measurement of urinary cystine levels 1
- Uric acid crystals suggest uric acid stone disease 4
- Calcium oxalate and calcium phosphate crystals may be seen with calcium-based stones 4, 6
Indicators of Infection-Related Stones
- Positive nitrites and leukocyte esterase with alkaline pH suggest struvite (infection) stones, which form in the presence of urease-producing bacteria 1, 3
- Urine culture should be obtained when urinalysis suggests infection, as struvite stones require different management than metabolic stones 1
Additional Urinalysis Components
Proteinuria Assessment
- Significant proteinuria suggests renal parenchymal disease rather than simple stone disease and warrants nephrology evaluation 2, 3
- Cellular casts or dysmorphic erythrocytes (>80% dysmorphic RBCs) indicate glomerular bleeding rather than stone-related bleeding 1, 2
Complete Urinalysis Requirements
- Both dipstick AND microscopic evaluation should be performed, as dipstick alone is insufficient for clinical decision-making 1, 3
- Microscopic examination should assess for RBCs, crystals, bacteria, casts, and cellular morphology 1, 7
- A freshly voided, clean-catch, midstream specimen is essential for accurate interpretation 8, 3
Critical Clinical Pitfalls
Common Mistakes to Avoid
- Never rely solely on dipstick results without microscopic confirmation, as this leads to false positives and unnecessary workups 3
- Do not dismiss hematuria in patients on anticoagulants or antiplatelet agents—these medications may unmask underlying stone disease but do not cause hematuria themselves 8, 2
- Avoid attributing hematuria solely to urinary tract infection without follow-up urinalysis after treatment, as stones and infection can coexist 3
When Stone Analysis is Available
- Stone composition analysis should be obtained at least once when a stone is available, as it directly informs preventive metabolic testing and treatment strategies 1
- Approximately 79% of stones are calcium-based (oxalate and phosphate), 16.5% are uric acid, and smaller percentages are struvite, cystine, or other compositions 4