Agreement between whole blood and plasma sodium measurements in profound hyponatremia

Resolving Pseudohyponatremia: Validation of Plasma Sodium on Radiometer ABL800 Blood Gas Analyzers for Immediate Reflex Testing

OBJECTIVE

To perform validation of plasma sodium on blood gas analyzers to reflexively correct erroneous measurements by ion-selective electrodes (ISEs).

METHODS

We compared remnant specimens of whole blood and plasma collected by lithium heparin vacutainer with normal protein concentrations and no lipemia. Whole-blood specimens were tested for sodium concentration on the ABL800 Flex blood gas analyzer, followed by centrifugation for plasma separation, and repeat sodium determination on an aliquot of the plasma only. Also, plasma specimens were analyzed by indirect ISE on the Cobas 8000 series and by direct ISE on the ABL800 Flex for instrument comparison.

RESULTS

Plasma aliquots yielded comparable results to the parent whole-blood specimen, with an average change of -1.33 mmol/L (R2 = 0.9727). Comparison of indirect ISE to direct ISE similarly yielded comparable results, with an average change of + 0.8 mmol/L (R2 = 0.9016).

CONCLUSION

Plasma is a valid specimen matrix for use on blood gas analyzers for sodium determination, eliminating the need for re-collection of whole-blood specimens from patients with pseudohyponatremia.

Graded interference with the direct potentiometric measurement of sodium by hemoglobin

OBJECTIVES

Sodium concentration is measured by either indirect (I_Na) or direct potentiometry (D_Na), on chemistry and gas panels, respectively. A spurious difference between these methods (ΔNa=I_Na-D_Na) can be confusing to the clinician. For example, variation in serum total protein (TP) is well known to selectively interfere with I_Na. Red cells have been suggested to interfere with D_Na, but both positive and negative interference have been reported. In this study, the effect of gas panel hemoglobin (Hb) on ΔNa was examined.

METHODS

ΔNa was calculated in 772 pairs of closely-timed chemistry and gas panels (median: 4min. apart), retrospectively collected from our critical care units, with 1 pair per patient. Hb was treated as a categorical or continuous variable and tested for linear and non-linear effects, with adjustment for 3 known influences on ΔNa-TP, bicarbonate (tCO₂), and the chemistry-gas panel glucose difference (ΔGlu).

RESULTS

Hb ranged from 3.5 to 22.0g/dL [35-220g/L]. In categorical analysis, ΔNa increased with Hb, and the effect was essentially linear. By simple regression, ΔNa rose 0.06±0.03[SE]mmol/L per 1g/dL [10g/L] increase in Hb (p<0.05), but confounding was suspected because Hb also correlated (p<10^-3) with TP, tCO₂, and ΔGlu. Using multiple regression to adjust for the confounders, ΔNa rose 0.15±0.03mmol/L per 1g/dL [10g/L] rise in Hb (p<10^-6).

CONCLUSIONS

Increasing Hb spuriously decreases D_Na and increases ΔNa. A linear correction for this artifact can reduce the discordance between I_Na and D_Na, promoting their interchangeable use.

Acute severe hyponatraemia secondary to polydipsia and associated herbal remedy use

A 47-year-old woman presented with seizures secondary to euvolaemic hyponatraemia. A collateral history revealed recent increased oral fluid intake and increased use of herbal remedies including valerian root over the New Year period. There was no history of psychiatric disease to support psychogenic polydipsia. She responded to careful sodium replacement in the intensive care unit and was discharged with no neurological sequelae.

Comparison of serum sodium levels measured by blood gas analyzer and biochemistry autoanalyzer in patients with hyponatremia, eunatremia, and hypernatremia

BACKGROUND

Blood gas analyzer (BGA) electrolyte measurements are frequently used in emergency departments (EDs) pending biochemistry laboratory autoanalyzer (BLA) results. There is lack of data in the literature in terms of agreement of these 2 measurement methods of sodium. We aimed to comprehensively evaluate the agreement in hyponatremia, eunatremia, and hypernatremia groups.

METHODS

Retrospectively, adult subjects who presented to ED of a tertiary care teaching hospital and had simultaneous BGA and BLA results were included in the study. Blood pairs were grouped into hyponatremia, eunatremia, and hypernatremia according to BLA results. Agreement of sodium measurements between the methods were evaluated by Bland-Altman plots and Passing and Bablok regression analysis.

RESULTS

A total of 2557 blood pairs (1326 males [51.8%]) were included. Median age of the patients was 66 years (18-103). The numbers of patients with hyponatremia, eunatremia, and hypernatremia were 487 (19%), 1943 (76%), and 127 (5%), respectively. The minimum and maximum serum sodium levels measured by biochemistry analyzer were 106 and 171 mmol/L, respectively. The Pearson linear correlation coefficient between BGA and BLA for sodium measurements were 0.574, 0.358, and 0.562 in hyponatremia, eunatremia, and hypernatremia groups, respectively. The absolute mean difference for the 3 groups was greater than 4 mmol/L. Biochemistry laboratory autoanalyzer tended to measure serum sodium higher than BGA in all sodium groups. Passing and Bablok regression analysis showed significant differences between the 2 methods in all sodium groups.

CONCLUSION

This is the first comprehensive evaluation of agreement between BGA and BLA in distinct sodium groups. Significant differences should be taken into account when these patients are managed in the ED.