Plasma oxalate and glycolate concentrations in dialysis patients with and without primary hyperoxaluria type 1

Graphical Abstract

Graphical Abstract.

Primary hyperoxaluria type 1 (PH1) is an inherited disorder that leads to endogenous overproduction of oxalate and glycolate. Overproduction of oxalate may lead to kidney failure, whereas glycolate is considered harmless [1, 2]. In kidney failure, elevated plasma oxalate (Pox) is the most important biomarker for PH1, but Pox may also be elevated in non-PH patients with kidney failure, which complicates its interpretation. Furthermore, different analytical methods result in poor comparison between centers [3]. Plasma glycolate (Pgly) may distinguish PH1 from other causes of kidney failure but has only been studied in very small numbers [4, 5].

Our primary aim was determining Pox and Pgly in non-PH dialysis patients and comparing these results to retrospectively collected data of PH1 patients with kidney failure. The preferred sample size was 120 non-PH dialysis patients [6]. After obtaining informed consent, we prospectively included 135 non-PH patients [80 male; 94 on hemodialysis (HD), 25 on peritoneal dialysis (PD), 16 pre-dialysis]. From patients who underwent HD three times a week, we obtained two samples, after both a normal and a long dialysis-free interval. Pre-analytical sample handling is described in our laboratory protocol [7]. Directly before analysis, samples were thawed. After addition of ethoxyamine and water, samples were incubated at 80°C for 30 min. After cooling and addition of an NaCl solution and ethyl acetate, samples were vortexed and centrifuged. The top layer was transferred into a clean gas chromatography vial. This extraction step was repeated once and the two top layers were combined. Analyses were performed with a 7890A gas chromatography coupled to a 7000 Triple Quadrupole Mass Spectrometer. The upper reference limit of normal (URL) were 6.8 and 22 µmol/L for Pox and Pgly, respectively. Statistical analyses (independent samples T-test, one-way analysis of variance and simple linear regression) were performed using IBM SPSS Statistics 26.

Median age of patients was 62 years [interquartile range (IQR) 50–70.5]. Most patients were of European (42.2%), Surinamese (23.0%) or African descent (17.0%). Causes of kidney failure included acquired glomerulopathy (23.7%), hypertension (23.0%), multifactorial disease (22.2%), diabetes (9.6%), unknown causes (5.2%), polycystic kidney disease (4.4%), structural abnormalities (3%) and other kidney diseases (8.8%). Median dialysis vintage was 3.5 years (IQR 1.7–10.0). Hemodiafiltration was used in 35.1% of patients. Anuric patients accounted for 59.6% of 94 patients on HD and for 16.0% of 25 patients on PD.

Median Pox concentrations were 46.5 µmol/L (range 16.9–89) and 50.9 mol/L (range 13.9–114.4) after a normal and long HD-free interval, respectively (Fig. 1), and 28.3 µmol/L (range 10.6–104.1) in PD patients. There were 48 patients on HD and 8 patients on PD with Pox >50 µmol/L, two of whom were prone to secondary hyperoxaluria. The other patients prone to secondary hyperoxaluria (n = 7) or those with oxalate crystals in kidney biopsies or nephrocalcinosis (n = 4) did not have Pox >50 µmol/L. There was a significant negative association between residual diuresis and Pox [F-statistic (1,91) = 27.53; P < .001 and R² = 0.23 in HD patients and F-statistic(1,19) = 6.57; P = .019 and R² = 0.26 in PD patients]. Pox was 52.0 µmol/L in anuric non-PH HD patients and decreased by 10.4 µmol/L (95% confidence interval 6.5–14.4) for every liter increase in residual diuresis; for PD patients these figures were 59.0 µmol/L and 17.2 µmol/L (95% confidence interval 3.2–31.3). For patients not on dialysis, median Pox was 7.2 µmol/L in eight patients with chronic kidney disease (CKD) stage 4 and 12.5 µmol/L in eight patients with CKD stage 5. Age, sex, diagnosis, dialysis vintage and technique were not found to influence Pox. Pgly was within the normal range in all patients. In 26 PH1 anuric patients on dialysis [14 males, median age 23 years (IQR 4–44)], median Pox was 143.9 µmol/L (range 13.7–399). Median dialysis vintage was 0.2 years (IQR 0.0–0.7). In nine patients, Pox was within the range of non-PH patients on HD. Median Pgly of 16 patients was elevated in all but one patient (median 161.5 µmol/L, range 7.6–681). Pox and Pgly were not correlated in PH1 patients.

Figure 1:

Plasma oxalate in patients with and without primary hyperoxaluria. Short refers to predialysis samples after a 1-day dialysis-free interval, long to sampling after a 2-day dialysis-free interval. Dots represent outliers.

Other studies in non-PH patients with CKD stages 4–5 have reported median Pox between 1 and 25 times the URL for individuals with normal kidney function. Two studies reported Pox values of 1–2× URL in patients in CKD stages 4–5 [8, 9], but patients on dialysis were not analyzed separately. In two other studies, median Pox in patients on dialysis were 28 and 44 µmol/L, corresponding to 9 and 25× URL, respectively [10, 11]. These studies all used enzymatic assays. Another study, using liquid chromatography/mass spectrometry in over 200 kidney transplant recipients, reported a median Pox of 35 µmol/L (3× URL) [12]. The mean serum creatinine of 600 µmol/L suggests residual diuresis in a large proportion of these patients. Differences between our findings and previously reported data could be explained by patient characteristics (e.g. residual diuresis, which accounted for 25% of variance in Pox) on one hand and methodological differences, which may result in a 33% bias [3], on the other hand.

In conclusion, our results challenge the assumption that Pox of 50 µmol/L is suggestive for PH1 [13], since this was the median value in our cohort of non-PH patients on HD. Residual diuresis was an important determinant and mainly accounted for differences between patients on HD and PD. Pox should thus be carefully interpreted using laboratory-specific reference values, adapted for dialysis patients and stratified for (the absence of) residual diuresis. Pgly was only elevated in PH1 patients, and thus is indicative of the disease. We advocate that a wider availability of glycolate measurements is useful for diagnostic purposes, notwithstanding the need for genetic testing to confirm the diagnosis.

New questions arise since the supersaturation threshold for Pox is assumed to be 40–50 µmol/L, a chemically determined concentration at which oxalate would start accumulating systemically [14]. This threshold was exceeded by nearly all included PH1 patients, but also by 56 non-PH patients on dialysis. Eight of them even reached concentrations above 80 µmol/L. It is unknown whether non-PH patients with this high Pox develop clinical manifestations of systemic oxalosis in the eyes, heart or bones. Further research is required and reevaluation of the supersaturation threshold may be needed.

CONFLICT OF INTEREST STATEMENT

All authors declare no conflicts of interest for the research, authorship and/or publication of this article.