Diet-related urine collections: assistance in categorization of hyperoxaluria

Hyperoxaluria, one of the major risk factors for calcium oxalate urolithiasis and nephrocalcinosis, causes significant morbidity and mortality and should therefore be detected and treated as soon as possible. An early, consequent and adequate evaluation, but also a distinction between primary (PH) and secondary hyperoxaluria (SH) is therefore essential. We evaluated the usefulness of three consecutive 24-h urine collections under different diets [usual diet, (A), low oxalate diet, (B), high oxalate diet, (C)] to prove SH, or to find evidence of PH by changes in urinary oxalate excretion (Uox). We retrospectively analyzed results from 96 pediatric patients (47 females and 49 males, age 3–18 years) who presented with a history of nephrolithiasis, nephrocalcinosis and/or persistent hematuria in whom hyperoxaluria was found in an initial urine sample. The typical pattern of SH was found in 34 patients (mean Uox (A) 0.85 ± 0.29, (B) 0.54 ± 0.15 and (C) 0.95 ± 0.28 mmol/1.73m²/d). PH was suspected in 13 patients [(A) 1.21 ± 0.75; (B) 1.47 ± 0.51 and (C) 1.60 ± 0.82 mmol/1.73m²/d], but genetically proven only in 1/5 patients examined. No hyperoxaluria was found in 16 patients. Data were inconclusive in 33 patients. Urine collection under different diets is helpful to diagnose secondary hyperoxaluria and may provide evidence, that urinary oxalate excretion is normal. We have now established this procedure as our first diagnostic step before further, more extensive and more expensive evaluations are performed.

Primary and secondary hyperoxaluria: Understanding the enigma

Hyperoxaluria is characterized by an increased urinary excretion of oxalate. Primary and secondary hyperoxaluria are two distinct clinical expressions of hyperoxaluria. Primary hyperoxaluria is an inherited error of metabolism due to defective enzyme activity. In contrast, secondary hyperoxaluria is caused by increased dietary ingestion of oxalate, precursors of oxalate or alteration in intestinal microflora. The disease spectrum extends from recurrent kidney stones, nephrocalcinosis and urinary tract infections to chronic kidney disease and end stage renal disease. When calcium oxalate burden exceeds the renal excretory ability, calcium oxalate starts to deposit in various organ systems in a process called systemic oxalosis. Increased urinary oxalate levels help to make the diagnosis while plasma oxalate levels are likely to be more accurate when patients develop chronic kidney disease. Definitive diagnosis of primary hyperoxaluria is achieved by genetic studies and if genetic studies prove inconclusive, liver biopsy is undertaken to establish diagnosis. Diagnostic clues pointing towards secondary hyperoxaluria are a supportive dietary history and tests to detect increased intestinal absorption of oxalate. Conservative treatment for both types of hyperoxaluria includes vigorous hydration and crystallization inhibitors to decrease calcium oxalate precipitation. Pyridoxine is also found to be helpful in approximately 30% patients with primary hyperoxaluria type 1. Liver-kidney and isolated kidney transplantation are the treatment of choice in primary hyperoxaluria type 1 and type 2 respectively. Data is scarce on role of transplantation in primary hyperoxaluria type 3 where there are no reports of end stage renal disease so far. There are ongoing investigations into newer modalities of diagnosis and treatment of hyperoxaluria. Clinical differentiation between primary and secondary hyperoxaluria and further between the types of primary hyperoxaluria is very important because of implications in treatment and diagnosis. Hyperoxaluria continues to be a challenging disease and a high index of clinical suspicion is often the first step on the path to accurate diagnosis and management.