Below you will find two board type questions to help with stone work-ups. If you are like me I often wonder what 24 hour urine tests I should order for a stone work up, how to interpret the results and what to do about the results. Hopefully these questions will help.
I think my 24 hr urine collections for a stone work up will include 24 hr urine volume, pH, sodium, potassium, creatinine, urea nitrogen, citrate, oxalate, uric acid, calcium
From KSAP
Question 16 of 75
A 39-year-old man is evaluated 4 months after undergoing a left ureteroscopic stone extraction for an 8-mm obstructing calculus. He has felt well since the procedure and is interested in preventing recurrent kidney stones. His father also has a history of nephrolithiasis.
The physical examination is unremarkable. Laboratory studies demonstrate normal serum electrolytes.
Additional data include the following:
Result Reference Range
Creatinine 1.0 mg/dL 0.7–1.3
Calcium 9.1 mg/dL 8.6–10.2
24-hour urine collection
Volume 2.9 L Varies with intake
pH 5.6 5.8–6.2
Sodium 170 mmol <150
Citrate 231 mg Male >450
Oxalate 37 mg <40
Uric acid 1029 mg <800
Calcium 265 mg Male <250
Stone analysis reveals 80% calcium oxalate monohydrate and 20% calcium oxalate dihydrate.
A follow-up kidney ultrasound shows two small nonobstructing stones measuring 2 and 3 mm in the right kidney and resolution of left-sided hydronephrosis.
In addition to chlorthalidone and general “stone clinic advice,” which of the following is the MOST appropriate additional treatment for this patient?
A Oxalobacter formigenes
B Potassium citrate
C Pyridoxine
D Sodium citrate
E Allopurinol
Answer & Explanation
Correct Answer is: B Potassium citrate
This patient’s risks for recurrent kidney stones include hypercalciuria and hypocituria, which should be addressed with a thiazide-like diuretic and potassium citrate supplementation.
Thiazide diuretics have been shown to significantly decrease that rate of recurrent stone disease in patients with hypercalciuria. However, thiazide diuretics are also associated with lower 24-hour urinary citrate excretion. A thiazide diuretic plus potassium citrate is the best management strategy to reduce the risk of recurrent stone formation because this man has both hypercalciuria and hypocitraturia. Potassium citrate is preferred to sodium citrate because an increase in sodium excretion will tend to increase calcium excretion, potentially off-setting some of the benefit of the chlorthalidone. In addition, the use of a thiazide is likely to cause hypokalemia and potassium citrate can offset these losses.
Allopurinol was demonstrated in a prospective randomized trial to reduce the risk of recurrent calcium oxalate stones. However, this risk reduction was in the setting of hyperuricosuria and normocalciuria. Therefore, allopurinol is not the best initial intervention for this patient.
Oxalabacter formigenes and pyridoxine are both used to treat hyperoxaluria. Oxalabacter formigenes is a bacterial species that metabolizes oxalate and is currently under investigation as a probiotic to reduce dietary oxalate absorption and urinary oxalate excretion in patients with enteric hyperoxalauria. Pyridoxine is a coenzyme of the hepatic peroxisomal enzyme alanine-glyoxylate aminotransferase (AGT), which is decreased or absent in patients with primary hyperoxalauria type 1. A subset of patients with primary hyperoxaluria type 1 may respond to high-dose pyridoxine therapy with a reduction in urinary oxalate excretion. Neither of these therapies would be indicated in this man who does not have hyperoxaluria.
All patients with kidney stones should be counseled to increase fluid intake throughout the day and limit sodium intake. In addition, an increase in fruits and vegetables and a reduction in animal protein intake can improve urinary citrate levels. Age-appropriate calcium intake rather than a low calcium diet is also recommended.
References
Zisman AL: Effectiveness of treatment modalities on kidney stone recurrence. Clin J Am Soc Nephrol 12(10): 1699–1708, 2017
Fink HA, Wilt TJ, Eidman KE, Garimella PS, MacDonald R, Rutks IR, Brasure M, Kane RL, Ouellette J, Monga M: Medical management to prevent recurrent nephrolithiasis in adults: A systemic review for an American College of Physicians Clinical Guideline. Ann Intern Med 158(7): 535–543, 2013
Pak CYC, Peterson R, Sakhaee K, Fuller C, Preminger G, Reisch J: Correction of hypocitraturia and prevention of stone formation by combined thiazide and potassium citrate therapy in thiazide-unresponsive hypercalciuric nephrolithiasis. Am J Med 79(3): 284–288, 1985
Ettinger B, Tang A, Citron JT, Livermore B, Williams T: Randomized trial of allopurinol in the prevention of calcium oxalate calculi. N Engl J Med 315(22): 1386–1389, 1986
Question 45 of 75
A 55-year-old woman is seen in follow-up for recurrent nephrolithiasis. She has passed approximately 20 kidney stones since age 30. Her physical examination is unremarkable. She weighs 65 kg. Laboratory studies show normal serum electrolytes and the following:
Result Reference Range
Creatinine 0.9 mg/dL 0.5–1.1
Calcium 9.5 mg/dL 8.6–10.2
Albumin 4 g/dL 3.5–5.5
24-hour urine collection
Volume 1.68 L Varies with intake
pH 5.9 5.8–6.2
Sodium 350 mmol <150
Potassium 100 mmol 20–100
Citrate 702 mg female >550
Oxalate 39 mg <40
Uric acid 1029 mg <800
Calcium 298 mg female <200
Uric acid 501 mg <1000
Creatinine 1050 mg 15–20 mg/kg
Urea nitrogen 9.14 g 6–14
Recent stone analysis revealed 100% calcium oxalate dihydrate.
In addition to increased fluid intake, which of the following is the MOST appropriate dietary intervention to reduce the risk of recurrent kidney stones?
A Reduced dairy and nondairy calcium intake
B Increased potassium intake
C Sodium restriction
D Decreased animal protein intake
E Increased citrate intake
Answer & Explanation
Correct Answer is: C Sodium restriction
After increasing her urine volume, dietary sodium restriction is the most important intervention for this patient with recurrent calcium oxalate kidney stones. She should be advised to reduce her sodium intake to <2 g (88 mmol) per day.
Her urine studies demonstrate low urine volume, hypercalciuria, and high sodium excretion as identifiable risk factors. Numerous studies have demonstrated that urinary calcium excretion is correlated with urinary sodium excretion and that hypercalciuria and risk for recurrent stones can be reduced by limiting dietary sodium intake. While she may ultimately benefit from thiazide therapy, an initial trial of dietary sodium restriction is warranted.
Lower calcium diets may reduce urinary calcium excretion, but are associated with a paradoxical increased risk of nephrolithiasis. Low calcium diets result in the loss of enteric calcium-oxalate binding and thereby lead to increased oxalate absorption and hyperoxaluria.
A high animal protein diet may cause hyperuricosuria and hypocitraturia. Approximately 16% of a mixed protein diet is converted to urea. Therefore, dietary protein intake (DPI) can be estimated by dividing the daily urea nitrogen excretion (in grams) by 0.16. In this case, DPI = 9.14 ÷ 0.16 = 57.1 g/day (or 0.88 g/kg per day). At steady state, normal protein intake and excretion should be approximately 0.8 g/kg per day. Thus, the 24-hour urine urea excretion does not suggest excessive protein intake.
Higher potassium diets, typically seen with consumption of fruits and vegetables, result in increased urinary citrate and have been demonstrated to reduce the risk of stone disease. In this case, both the 24-hour urine citrate and potassium excretion indicate that her dietary intake of potassium and citrate are adequate, so supplementation of either of these would not be the next best step in therapy.
References
Ticinesi A, Nouvenne A, Maalouf NM, Borghi L, Meschi T: Salt and nephrolithiasis. Nephrol Dial Transplant 31: 39–45, 2016
Curhan GC, Willett WC, Speizer FE, Spiegelman D, Stampfer MJ: Comparison of dietary calcium with supplemental calcium and other nutrients as factors affecting the risk for kidney stones in women. Ann Intern Med 126(7): 497–504,1997
Sorensen MD, Kahn AJ, Reiner AP, Tseng TY, Shikany JM, Wallace RB, Chi T, Wactawski-Wende J, Jackson RD, O’Sullivan MJ, Sadetsky N, SToller ML; WHI Working Group: Impact of nutritional factors on incident stone formation: A report from the WHI. J Urol 187(5): 1645–1649, 2012
Penniston K, Steele TH, Nakada SY: Lemonade therapy increases citrate and urine volumes in patients with recurrent calcium oxalate stone formation. Urology 70(5): 856–860, 2007
Muldowney FP, Freaney R, Moloney MF: Importance of dietary sodium in the hypercalciuria syndrome. Kidney Int 22(3): 292–296, 1982