From the Guidelines
The next step in the workup for a patient with calcium oxalate crystals on urinalysis should be a comprehensive metabolic evaluation, including a detailed medical and dietary history, serum chemistries, and urinalysis, as recommended by the American Urological Association guideline 1. This evaluation should include a screening of the patient's medical and dietary history to identify any conditions, habits, or medications that may predispose to stone disease, such as low fluid intake, high sodium intake, or limited intake of fruits and vegetables.
- A detailed history should elicit from the patient their average daily intake of fluids, protein, calcium, sodium, high oxalate-containing foods, fruits and vegetables, and over-the-counter supplements.
- Serum chemistries should include electrolytes, calcium, creatinine, and uric acid to suggest underlying medical conditions associated with stone disease.
- Urinalysis should include both dipstick and microscopic evaluation to assess urine pH and indicators of infection and to identify crystals pathognomonic of stone type. Additionally, patients should be queried regarding their regular use of any stone-provoking medications or supplements, and urine culture should be obtained in patients with a urinalysis suggestive of urinary tract infection or in patients with recurrent UTIs 1. It is also important to note that patients with recurrent calcium oxalate stones who have hyperuricosuria and normal urinary calcium may benefit from allopurinol therapy, as demonstrated by a prospective randomized controlled trial 1.
- However, the primary focus should be on identifying and addressing the underlying metabolic abnormalities that contribute to stone formation, rather than solely on medical management. By taking a comprehensive approach to the workup and management of patients with calcium oxalate crystals, clinicians can reduce the risk of recurrence and improve patient outcomes.
From the Research
Next Steps in Workup for Calcium Oxalate Crystals
The identification of calcium oxalate crystals on urinalysis indicates a potential risk for kidney stone formation. To further evaluate and manage this condition, the following steps can be taken:
- Analysis of stone composition by polarization microscopy, if a stone is available 2
- Measurement of serum calcium, phosphate, uric acid, 1,25(OH)2D3, and creatinine to assess for underlying metabolic disorders 2
- 24-hour urine collection to analyze volume, pH, and excretion of calcium, phosphorus, magnesium, uric acid, citrate, sodium, oxalate, and creatinine 2, 3, 4
- Evaluation of medical history, including dietary and drinking habits, as well as lifestyle, to identify potential risk factors for stone formation 3
- Identification of specific medical conditions, such as hypercalciuria or hyperoxaluria, which may be contributing to stone formation 3, 5
- Assessment of urine saturation and risk factors for calcium oxalate stone disease using spot and 24-hour urine specimens 4
Diagnostic Considerations
When evaluating a patient with calcium oxalate crystals on urinalysis, it is essential to consider the following:
- The type of stone and its composition, as this can influence treatment and prevention strategies 3, 6
- The severity of the disease, as this can impact the aggressiveness of treatment and the need for ongoing monitoring 3
- The presence of underlying medical conditions, such as primary or secondary hyperoxaluria, which can increase the risk of stone formation and oxalate nephropathy 5
Treatment and Prevention
Treatment and prevention strategies for calcium oxalate stones may include:
- Increasing urine volume and reducing urine calcium to below 200 mg/24 hr with thiazide 2
- Maintaining dietary calcium intake at 600 to 800 mg/day and adding potassium citrate if urine citrate levels are reduced 2
- Reducing urine oxalate excretion through dietary oxalate restriction, if elevated 2
- Using pharmacological agents, such as calcium supplements, to reduce the risk of stone formation 5