Calcium Oxalate Nephrolithiasis

Calcium oxalate nephrolithiasis is stone formation from calcium oxalate monohydrate (whewellite, dense) and dihydrate (weddellite), the most common composition (70 to 80 percent of stones).

• Urinary risk factors: low volume, hypercalciuria (above 7 mmol/day in men and 6 mmol/day in women), hyperoxaluria (above 0.5 mmol/day), hypocitraturia (below 1.7 mmol/day in men, below 2.5 mmol/day in women), hyperuricosuria, low magnesium, acidic urine.
• Causes: primary hyperparathyroidism, idiopathic hypercalciuria, enteric hyperoxaluria (Crohn disease, short bowel, bariatric surgery, pancreatic insufficiency), primary hyperoxaluria, high oxalate foods, high dose vitamin C, low calcium diet, high sodium and animal protein, dehydration (hot climate), distal renal tubular acidosis, sarcoidosis, medullary sponge kidney.

Renal colic, haematuria, flank pain, urinary tract infection, or asymptomatic stones found on imaging.

• Recurrent stones, bilateral stones, early onset in childhood or adolescence suggests metabolic disease.
• Differential diagnosis: other stone types (uric acid, struvite, cystine, calcium phosphate), papillary necrosis, tumour, infection, nephrocalcinosis.
• Complications: obstruction, infection, acute kidney injury, chronic kidney disease (recurrent stones, primary hyperoxaluria with systemic oxalosis), hypertension, bone loss.

Stone analysis (infrared spectroscopy) after every stone.

• Serum calcium, albumin, phosphate, parathyroid hormone (if calcium high or high normal), urate, creatinine, bicarbonate, potassium, vitamin D.
• Urine: pH, culture, and two 24-hour collections (volume, calcium, oxalate, citrate, uric acid, sodium, magnesium, phosphate, creatinine) at least 6 weeks after a stone episode.
• Low dose CT to count stones and monitor; ultrasound as a follow-up.
• Gut investigation for malabsorption if enteric hyperoxaluria; genetic testing for primary hyperoxaluria.

Supportive (applies to all stone formers):

• Fluids to achieve urine output of 2.5 L daily (at least 3 L fluid; more in hot climates), spread through the day and night; water preferably.
• Dietary calcium 1000 to 1200 mg daily (do not restrict calcium); sodium below 2 g daily (about 5 g of salt); animal protein 0.8 to 1 g/kg/day; reduce oxalate foods (spinach, rhubarb, nuts, tea in large amounts, chocolate, beetroot); avoid vitamin C supplements above 500 mg daily; maintain a healthy weight; moderate fructose and sugar sweetened drinks.

Pharmacological (tailored to 24-hour urine):

• Hypercalciuria: hydrochlorothiazide 25 mg twice daily (or 50 mg daily), chlorthalidone 25 mg daily, or indapamide 2.5 mg daily, with potassium monitoring and sodium restriction.
• Hypocitraturia: potassium citrate 30 to 60 mmol daily in 2 to 3 divided doses, titrated to urine pH 6.0 to 7.0 and citrate above 2 mmol/day (alternatively lemon juice 120 mL diluted daily, sodium bicarbonate 1 g three times daily).
• Hyperuricosuria: allopurinol 100 mg increasing to 300 mg daily.
• Hyperoxaluria: dietary oxalate restriction and calcium 500 mg with meals; pyridoxine 100 to 200 mg daily for mild primary hyperoxaluria (up to 5 mg/kg/day in type 1); enteric hyperoxaluria: low fat diet, calcium carbonate 1 to 2 g with meals, cholestyramine 4 g with meals, potassium citrate.
• Primary hyperoxaluria type 1: lumasiran where available.
• Hypomagnesuria: magnesium oxide 400 mg twice daily.

Procedures: parathyroidectomy for primary hyperparathyroidism (calcium above 2.6 mmol/L); definitive stone removal by lithotripsy, ureteroscopy or percutaneous nephrolithotomy.

Special groups: children (genetic work-up), pregnancy (avoid thiazides if possible, hydration), chronic kidney disease (avoid thiazides if eGFR is below 30 mL/min).

Red flags: obstruction with infection, solitary kidney, nephrocalcinosis, bilateral stones in children, rising creatinine.

Follow-up: repeat 24-hour urine 8 to 12 weeks after starting therapy then annually; imaging yearly for 2 years; serum calcium, potassium and creatinine every 6 to 12 months.