Interchangeability of sodium and chloride measurements by indirect and direct ISE assays: Stakeholders, take responsibility!

Analytical evaluation of a direct ion-selective-based analyser: Still gaps to close

INTRODUCTION

Discrepancies between electrolyte concentrations determined by blood gas analysers (BGA) and core-lab chemistry analysers may create confusion in clinical practice. This problem is rooted in the different ion-selective electrode (ISE) methodologies that are used. Whilst most available chemistry analysers use indirect ISE, we evaluated the analytical performance of the new automated chemistry analyser Biossays™ E6 (Snibe), equipped with direct ISE, for the determination of sodium (Na+), potassium (K+), chloride (Cl-), ionized calcium (iCa2+) and pH.

MATERIALS AND METHODS

Total precision, estimated deviation and total error were evaluated for all analytes on the E6 analyser. Several patient cohorts were used to perform method comparisons between the E6 and the direct (RP500e BGA) and indirect (Architect c16000 analyser) ISE methods routinely used in the lab. Obtained data were compared against pre-set quality specifications and used for adjustment of the 2 direct ISE methods. For Na+ and iCa2+, agreement with the routinely used protein-corrected Na+ and total calcium (TCa2+) concentrations were assessed respectively.

RESULTS

The analytical performance for the 4 tested electrolytes (Na+, K+, Cl-, iCa2+) and pH were acceptable and within the specified performance specifications. After adjustment of both direct methods, method comparison on an independent patient cohort showed good agreement. For Na+ and iCa2+, a good correlation with the protein corrected Na+ and TCa2+ results was observed.

CONCLUSION

The acceptable analytical performance and ease-of-use of the E6 direct ion selective instrument is making it feasible to optimize electrolyte determinations to direct methodology.

Paraproteins and electrolyte assays: exclusion effect and effect of paraprotein elimination

Paraproteins are a potential source of error for electrolyte analyses. The exclusion effect itself causes a discrepancy between direct and indirect ion selective electrode assays (dISE and iISE, respectively). We tested the applicability of different pretreatment methods and the difference of dISE and iISE with paraprotein-rich samples. We analysed chloride (Cl-), potassium (K+), and sodium (Na+) on 46 samples with paraproteins up to 73 g/L. We compared pretreatment methods of preheating, precipitation, and filtration to the native sample. All induced a statistically significant difference (p-value <0.05). Clinically significant difference was induced by precipitation for all analytes, and filtration for Cl- and Na+, but for none by preheating. The difference in electrolyte measurements with either dISE or iISE on native samples was explained by total protein concentration (TP). There was a statistically significant difference in all electrolyte measurements. On average, there was a clinically significant difference in Na + but not in Cl- and K + measurements. Paraprotein concentration (PP) or heavy chain class did not induce a statistically significant effect. The regression analysis and comparison to the theoretical exclusion effect supported the conclusion that TP is the only explanatory factor in the difference between dISE and iISE. We conclude that preheating is a suitable pretreatment method for all the studied analytes. Precipitation is not valid for any of them, and filtration can be considered only for K+. Because the difference between dISE and iISE was explained by the exclusion effect caused by TP, dISE is the more suitable method to analyse paraprotein-rich samples.

Investigating the effects of endogenous lipaemia on the measurement of sodium by indirect ion specific electrode potentiometry

BACKGROUND

The widely automated method using indirect ion specific electrodes (ISE) potentiometry for determination of sodium concentration is prone to interference from lipaemia. Manufacturer-specified lipaemic (L)-index cut offs may underestimate the effects of endogenous lipaemia.

METHODS

We assessed the interference on sodium concentration caused by endogenous lipaemia in 32 residual samples (from 13 patients) using indirect ISE (Cobas^® 8000 modular analyser with c702 module, Roche diagnostics) and direct ISE (GEM 4000 premier, Werfen) potentiometric methods. Regression analysis (linear and non-linear) was used to determine a reliable (L)-index cut off for reporting sodium concentration.

RESULTS

There was a poor correlation observed between triglyceride concentration and (L)-index. There was significant negative interference caused by endogenous lipaemia within analysed samples. Non-linear regression demonstrated a negative interference of approximately 5% at an (L)-index of 250.

CONCLUSION

At present, the manufacturer advises not to report sodium concentration by indirect ISE on the Cobas^® 8000 modular analyser if the (L)-index is >2000. However, this has been determined by the addition of exogenous lipids (Intralipid^®) and it is clear that this is not comparable to endogenous lipaemia. To ensure patient safety, clinical laboratories should consider lowering the cut off for (L)-index that they use for reporting sodium concentration.

Interference in Ion-Selective Electrodes Due to Proteins and Lipids

BACKGROUND

Ion-selective electrodes (ISE) have become the mainstay of electrolyte measurements in the clinical laboratory. In most automated analyzers used in large diagnostic laboratories, indirect ISE (iISE) -based electrolyte estimation is done; whereas direct ISE (dISE) -based equipment are mostly used in blood gas analyzers and in the point-of-care (PoC) setting.

CONTENT

Both the techniques, iISE as well as dISE, are scientifically robust; however, the results are often not interchangeable. Discrepancy happens between the two commonly due to interferences that affect the two measuring principles differently. Over the last decade, several studies have reported discrepancies between dISE and iISE arising due to abnormal protein and lipid contents in the sample.

SUMMARY

The present review endeavors to consolidate the knowledge accumulated in relation to interferences due to abnormal protein and lipid contents in sample with the principal focus resting on probable solutions thereof.

Association Between an Increase in Serum Sodium and In-Hospital Mortality in Critically Ill Patients

Supplemental Digital Content is available in the text.

OBJECTIVES:

In critically ill patients, dysnatremia is common, and in these patients, in-hospital mortality is higher. It remains unknown whether changes of serum sodium after ICU admission affect mortality, especially whether normalization of mild hyponatremia improves survival.

DESIGN:

Retrospective cohort study.

SETTING:

Ten Dutch ICUs between January 2011 and April 2017.

Patients:

Adult patients were included if at least one serum sodium measurement within 24 hours of ICU admission and at least one serum sodium measurement 24–48 hours after ICU admission were available.

INTERVENTIONS:

None.

MEASUREMENTS AND MAIN RESULTS:

A logistic regression model adjusted for age, sex, and Acute Physiology and Chronic Health Evaluation-IV–predicted mortality was used to assess the difference between mean of sodium measurements 24–48 hours after ICU admission and first serum sodium measurement at ICU admission (Δ48 hr-[Na]) and in-hospital mortality. In total, 36,660 patients were included for analysis. An increase in serum sodium was independently associated with a higher risk of in-hospital mortality in patients admitted with normonatremia (Δ48 hr-[Na] 5–10 mmol/L odds ratio: 1.61 [1.44–1.79], Δ48 hr-[Na] > 10 mmol/L odds ratio: 4.10 [3.20–5.24]) and hypernatremia (Δ48 hr-[Na] 5–10 mmol/L odds ratio: 1.47 [1.02–2.14], Δ48 hr-[Na] > 10 mmol/L odds ratio: 8.46 [3.31–21.64]). In patients admitted with mild hyponatremia and Δ48 hr-[Na] greater than 5 mmol/L, no significant difference in hospital mortality was found (odds ratio, 1.11 [0.99–1.25]).

CONCLUSIONS:

An increase in serum sodium in the first 48 hours of ICU admission was associated with higher in-hospital mortality in patients admitted with normonatremia and in patients admitted with hypernatremia.