Plasma oxalate in relation to eGFR in patients with primary hyperoxaluria, enteric hyperoxaluria and urinary stone disease

Background: Since plasma oxalate (POx) concentrations increase at lower glomerular filtration rate (GFR) levels, even among those without enteric (EH) or primary hyperoxaluria (PH), the appropriate thresholds for considering a disorder of oxalate metabolism are poorly defined. The current study was completed to establish relationships between POx, GFR, and urine oxalate excretion (UOx) among patients with PH, EH, and routine urinary stone disease (USD). Methods: The most recent POx measurement on all Mayo Clinic patients between 2005 and 2015 were electronically pulled from the Lab Information System together with the closest serum creatinine within 14 days and 24 h urine study within 60 days. After exclusion of patients not in steady state at the time of blood draw, 270 patients were available for study. Records were reviewed for clinical diagnoses to categorize patients as PH, EH, or USD. Waste plasma for Pox was also obtained from controls without USD undergoing clinical GFR testing. Results: In all 3 groups POx increased as eGFR fell. For any given eGFR, POx was highest in the PH group and lowest in the USD and control groups (p < 0.0001). POx was also influenced by UOx excretion (reflecting total body oxalate burden, absorption from diet and endogenous production). Generalized estimating equations of POx vs eGFR revealed higher average POx levels in PH compared to EH, USD or control, and for EH compared to USD or control. GEE prediction models were created that use POx, UOx, age, and serum creatinine to estimate the probability of a PH diagnosis. Conclusions: New models were developed to help interpret POx when considering PH in clinical practice even when it was not previously suspected and/or eGFR is reduced.