Electrolyte measurement - myths and misunderstandings- Part II

Effects of topiramate therapy on serum bicarbonate concentration in a sample of 10,279 veterans

BACKGROUND

Topiramate, which is increasingly being used to treat alcohol use disorder (AUD), is commonly associated with reduced serum bicarbonate concentrations. However, estimates of the prevalence and magnitude of this effect are from small samples and do not address whether topiramate's effects on acid-base balance differ in the presence of an AUD or by topiramate dosage.

METHODS

Veterans Health Administration electronic health record (EHR) data were used to identify patients with a minimum of 180 days of topiramate prescription for any indication and a propensity score-matched control group. We differentiated patients into two subgroups based on the presence of a diagnosis of AUD in the EHR. Baseline alcohol consumption was determined using Alcohol Use Disorders Identification Test-Consumption (AUDIT-C) scores in the EHR. Analysis also included a three-level measure representing mean daily dosage. The topiramate-associated changes in serum bicarbonate concentration were estimated in difference-in-differences linear regression models. A serum bicarbonate concentration <17 mEq/L was considered to represent possible clinically significant metabolic acidosis.

RESULTS

The cohort comprised 4287 topiramate-treated patients and 5992 propensity score-matched controls with a mean follow-up period of 417 days. The mean topiramate-associated reductions in serum bicarbonate concentration were <2 mEq/L in the low (≤88.75), medium (>88.75 and ≤141.70), and high (>141.70) mg/day dosage tertiles, irrespective of AUD history. Concentrations <17 mEq/L occurred in 1.1% of topiramate-treated patients and 0.3% of controls and were not associated with alcohol consumption or an AUD diagnosis.

CONCLUSIONS

The excess prevalence of metabolic acidosis associated with topiramate treatment does not differ with dosage, alcohol consumption, or the presence of an AUD. Baseline and periodic serum bicarbonate concentration measurements are recommended during topiramate therapy. Patients prescribed topiramate should be educated about the symptoms of metabolic acidosis and urged to report their occurrence promptly to a healthcare provider.

Impact of Bicarbonate Interference on Routine Ion-Selective Electrode Chloride Measurements

Background

Chloride measurement is crucial for calculating the anion gap. Bicarbonate can interfere with chloride measurements; however, there is no information on the specific types of electrodes affected or the changes in bicarbonate non-selectivity over time. We evaluated the interference of bicarbonate on chloride measurements using different electrodes and the stability of this interference over time.

Methods

The effect of bicarbonate on chloride measured with electrodes of various manufacturers was assessed. When non-selectivity toward bicarbonate was observed, the stability of this interference during the electrode's lifetime was explored. The impact of the bicarbonate concentration on the calibrator was also evaluated.

Results

Non-selectivity was observed for electrodes using quaternary ammonium salts (Beckman Coulter, Siemens, and Roche), with overestimated or underestimated chloride values observed at high or low bicarbonate concentrations, respectively. The degree of selectivity varied among electrodes. With the Roche electrode, interference became more pronounced over time, whereas the Siemens electrode appeared to gain selectivity during its lifetime. For the Roche system, adjusting the calibrator's bicarbonate concentration from 30 mmol/L to 20-24 mmol/L reduced the number of samples with unacceptable bias (>3%) from 77.3% to 12.6%. Lot-to-lot variations in the calibrator bicarbonate concentration increased the uncertainty of chloride measurements.

Conclusions

The extent of bicarbonate-induced error varied according to the type, manufacturer, and wear of the electrode; the bicarbonate concentration in the calibrators and the tested sample; and the chloride content. Laboratories should be aware of the impact of bicarbonate on routine chloride measurements to establish appropriate QC procedures.