Which laboratory technique is used for the blood sodium analysis in clinical research? A systematic review

Correlation of plasmatic sodium determined by the laboratory and that determined by the dialysis machine

INTRODUCTION

Changes in plasma sodium concentration (pNa, expressed in mEq/L) are common in hemodialysis (HD) patients. Hemodialysis monitors can estimate pNa by using an internal algorithm based on ion dialysance measurements. The present study studies the accuracy of the correlation between the pNa estimated by the dialysis monitor and that measured by the biochemistry laboratory at our center.

MATERIAL AND METHODS

A single-centre prospective observational study in patients on a chronic HD program with the 6008 CAREsystem monitor and standard sodium (138mmol/L) and bicarbonate (32mmol/L) prescriptions. Venous blood samples were drawn from each patient before and after each HD session to ensure inter- and intra-individual validity. The pNa was measured in the biochemistry laboratory using indirect potentiometry and simultaneously the estimated pNa by the HD monitor was recorded at the beginning and at the end of the HD session. For statistical analysis, a scatterplot was made, and Spearman's correlation quotient was calculated. In addition, the differences between both methods were represented as Bland-Altman diagrams.

RESULTS

The pre-dialysis pNa measured in the laboratory was 137.49±3.3, and that of the monitor, 137.96±2.91, with a correlation with R2 value of 0.683 (p<0.001). The post-dialysis pNa measured in the laboratory was 137.08±2.23, and that of the monitor was 138.87±1.88, with an R2 of 0.442 (p<0.001). On the Bland-Altman plots, the pre-dialysis pNa has a systematic error of 0.49, in favor of the monitor-estimated pNa, with a 95% confidence interval (CI) of (-3.24 to a 4.22). In the post-dialysis pNa, a systematic error of 1.79 with a 95% CI of (-1.64 to 5.22) was obtained.

CONCLUSION

The correlation between the pNa estimated by Fresnius 6008 CAREsystem HD monitor and that measured by the laboratory is good, especially pre-dialysis measurements. Further studies should verify the external validity of these results.

Hyponatremia and other potential markers of ultrasound abnormalities after a first febrile urinary tract infection in children

Urinary tract infections are the initial manifestation in 30% of urinary tract malformations. Identifying these patients, who could benefit from a specific treatment, is still challenging. Hyponatremia during urinary tract infection has been proposed as a urinary tract malformation marker. We evaluate the prevalence of hyponatremia during febrile urinary tract infections and its association with subjacent urinary tract malformations. We performed a retrospective study of healthy patients under 16 years, diagnosed with a first episode of febrile urinary tract infection, who had undergone blood testing in the acute episode and at least one renal ultrasound during follow-up (January 2014-November 2020). Hyponatremia was defined as (serum sodium ≤ 130 mEq/L). According to imaging findings, we classified patients into three groups: normal kidney ultrasound, mild pelviectasis, and significant urinary tract malformation. We performed logistic regression models to identify independent risk factors for urinary tract malformation and mild pelviectasis. We included 492 patients and 2.8% presented hyponatremia. We identified normal ultrasound in 77%, mild pelviectasis in 10.8%, and urinary tract malformation in 12% of patients. We found an association between mild pelviectasis and hyponatremia [OR 6.6 (CI95% 1.6-26.6)]. However, we found no association between hyponatremia and urinary tract malformation. The parameters that were associated with malformations were presenting a non-E. coli infection, C-reactive-protein levels over 80 mg/L, and bacteremia.

CONCLUSION

Hyponatremia during the first episode of febrile urinary tract infection is present in 2.8% of patients and is associated with mild pelviectasis in imaging. However, hyponatremia does not indicate a greater need for complementary tests to screen for urinary tract malformations.

WHAT IS KNOWN

• Urinary tract infection is the first manifestation in 30% of children with urinary tract malformation. • Hyponatremia could be a marker to identify these children and guide the imaging approach.

WHAT IS NEW

• Around 12% of children with a first episode of febrile urinary tract infection have a urinary tract malformation. • Non-E. coli infection, C-reactive protein levels over 80 mg/L, and bacteremia are markers for malformations to guide diagnostic imaging tests, but hyponatremia (Na ≤ 130 mEq/l) is not a reliable marker.

Dysnatremia and subsequent sodium level changes following various intravenous treatments in infants with acute gastroenteritis

Acute gastroenteritis is one of the main causes of electrolyte imbalance in infants. We aimed to determine the frequency of and factors associated with dysnatremia at presentation and establish the ideal intravenous treatment scheme. The records of hospitalized infants aged 1-12 months with community-acquired acute gastroenteritis between January 2017 and March 2021 were retrospectively reviewed. Factors associated with dysnatremia at presentation were analyzed by multivariable logistic regression analysis. Subsequent sodium levels 4-24 h after intravenous fluid treatments, which were categorized into 2 groups, were determined in the subgroup of infants with normal sodium levels at presentation. A total of 347 infants with a median age of 8.0 (5.0-10.0) months were included. The frequency of dysnatremia at presentation was 14% (hyponatremia 12% and hypernatremia 2.0%). Severe dehydration was associated with dysnatremia at presentation (p = 0.048). Among 68 infants with normal sodium levels at presentation, the median sodium change was highest in the 5% dextrose in saline group, with changes of + 3 (0.5-5) and + 1 (- 2 to 2) mmol/L in infants who received 5% dextrose in saline and 5% dextrose in 1/3-1/2 saline, respectively (p = 0.001). Four out of 47 infants (8.5%) developed hyponatremia while receiving 5% dextrose in 1/3-1/2 saline. None of those who received 5% dextrose in saline developed subsequent dysnatremia.   Conclusion: The frequency of dysnatremia at presentation among infants with acute gastroenteritis was 14%. Severe dehydration was associated with dysnatremia at presentation, so electrolyte levels need to be assessed in these patients. The use of isotonic solution did not promote acquired dysnatremia. This study supports once more that current guidelines recommending isotonic solution for children, and, especially, infant rehydration, are important also for infants in Thailand. What is Known: • There were a wide variation in the incidence of dysnatremia at presentation in children with acute gastroenteritis in previous pediatric series. • The AAP guidelines recommend using isotonic solution in children with acute illness from 28 days to 18 years of age to prevent acquired hyponatremia. What is New: • The incidence of dysnatremia at presentation in infants with acute gastroenteritis was 14% (hyponatremia 12% and hypernatremia 2.0%). • The use of isotonic solution did not promote acquired dysnatremia in infants with acute gastroenteritis.

The prevalence and outcomes of hyponatremia in children with COVID-19 and multisystem inflammatory syndrome in children (MIS-C)

Introduction

To assess the prevalence of hyponatremia among pediatric patients with coronavirus disease 2019 (COVID-19) and Multisystem Inflammatory Syndrome in Children (MIS-C) and determine if pediatric hyponatremia was associated with an increased length of stay, higher rates of mechanical ventilation, and/or elevated inflammatory markers on admission as compared to eunatremic patients.

Methods

Electronic health records were retrospectively analyzed for 168 children less than 18 years old with COVID-19 or MIS-C who were admitted to pediatric units within the Northwell Health system. The primary exposure was hyponatremic status (serum sodium <135 mEq/L) and the primary outcomes were length of stay, mechanical ventilation usage and increased inflammatory markers.

Results

Of the 168 children in the study cohort, 95 (56%) were admitted for COVID-19 and 73 (43.5%) for MIS-C. Overall, 60 (35.7%) patients presented with hyponatremia on admission. Patients with hyponatremia had higher rates of intensive care unit admission when compared to eunatremic patients (32/60 [53.3%] vs. 39/108 [36.1%], p = 0.030). In regression models, hyponatremia was not significantly associated with increased length of stay or mechanical ventilation rates. After adjustment for relevant confounders, hyponatremia remained associated with an increased square root CRP (β = 1.79: 95% CI: 0.22-3.36) and lower albumin levels (β = -0.22: 95% CI: -0.42--0.01).

Conclusion

Hyponatremia is common in pediatric COVID-19 and MIS-C. Hyponatremia was associated with a lower albumin and higher square root CRP levels. This may suggest an association of inflammation with lower serum sodium levels.

Pseudohyponatremia: Mechanism, Diagnosis, Clinical Associations and Management

Pseudohyponatremia remains a problem for clinical laboratories. In this study, we analyzed the mechanisms, diagnosis, clinical consequences, and conditions associated with pseudohyponatremia, and future developments for its elimination. The two methods involved assess the serum sodium concentration ([Na]_S) using sodium ion-specific electrodes: (a) a direct ion-specific electrode (ISE), and (b) an indirect ISE. A direct ISE does not require dilution of a sample prior to its measurement, whereas an indirect ISE needs pre-measurement sample dilution. [Na]_S measurements using an indirect ISE are influenced by abnormal concentrations of serum proteins or lipids. Pseudohyponatremia occurs when the [Na]_S is measured with an indirect ISE and the serum solid content concentrations are elevated, resulting in reciprocal depressions in serum water and [Na]_S values. Pseudonormonatremia or pseudohypernatremia are encountered in hypoproteinemic patients who have a decreased plasma solids content. Three mechanisms are responsible for pseudohyponatremia: (a) a reduction in the [Na]_S due to lower serum water and sodium concentrations, the electrolyte exclusion effect; (b) an increase in the measured sample's water concentration post-dilution to a greater extent when compared to normal serum, lowering the [Na] in this sample; (c) when serum hyperviscosity reduces serum delivery to the device that apportions serum and diluent. Patients with pseudohyponatremia and a normal [Na]_S do not develop water movement across cell membranes and clinical manifestations of hypotonic hyponatremia. Pseudohyponatremia does not require treatment to address the [Na]_S, making any inadvertent correction treatment potentially detrimental.

Outdoor temperature and circulating sodium in children with acute gastroenteritis

BACKGROUND

Dysnatremias are frequent in acute gastroenteritis. High outdoor temperatures have been associated with hyponatremia in both adults and the elderly, but no data are available among children with gastroenteritis.

METHODS

Children <10 years of age admitted to the emergency department of the Policlinico Hospital, Milan (Italy) between 2009 and 2019 with acute moderate-severe gastroenteritis were enrolled. The association between hyponatremia (sodium < 135 mmol/L) and daily mean levels of temperature or apparent temperature from day of admission up to 14 days before was analyzed by multivariable logistic regression models.

RESULTS

In 202 included children (46% females), we observed an increased risk of hyponatremia per unit increase in outdoor temperature of the sixth, eighth and ninth day before admission [Odds Ratio = 1.24 (95% Confidence Interval: 1.04-1.47), 1.14 (1.01-1.28), and 1.14 (1.01-1.28), respectively]. Analyses considering average temperature levels of the ten days preceding admission returned similar findings as well as those on apparent temperature.

CONCLUSIONS

Our data suggest the existence of an association between outdoor temperature and circulating sodium levels in children with acute gastroenteritis. The role of meteorological conditions on electrolyte imbalance should be further explored in the context of climate change.

IMPACT

The role of meteorological variables in the development of dysnatremias has been demonstrated in children and adolescents with cystic fibrosis. This study shows for the first time that higher outdoor temperatures are associated with a higher risk of hyponatremia in children with gastroenteritis. In the context of climate change, the role of external climate conditions on the risk of electrolyte imbalance in children should be more and more considered and explored in future studies.

Sodium assessment in neonates, infants, and children: a systematic review

Hyponatremia is a common disorder in childhood. The indirect and the direct potentiometry are currently the most popular techniques employed for sodium assessment, although discrepancies between the two techniques may be > 10 mmol/L. It is known that < 20% of the recently published articles report information about the technique used for sodium analysis, but no data are available on pediatric studies. This study aimed at investigating the laboratory technique employed for sodium measurement in studies conducted in childhood. A systematic literature search in PubMed, Embase, and Web of Science was undertaken to identify articles containing the word "hyponatremia" in the title between 2013 and 2020. Papers with < 10 subjects were excluded. A total of 565 articles were included. Information on the laboratory technique used for sodium analysis was more commonly (p = 0.035) reported in pediatric (n = 15, 28%) than in non-pediatric (n = 81, 16%) reports. The frequency of reports with and without information on the technique for sodium assessment was not different with respect to the study characteristics, the quartile of the journal where the paper was published, the country income setting, and the inclusion of neonates among the 54 pediatric studies.   Conclusion: Most pediatric papers do not report any information on the technique used for sodium analysis. Although international authorities have recommended the implementation of direct potentiometry, a low awareness on this issue is still widespread in pediatric research. What is Known: • Direct potentiometry and indirect potentiometry are currently employed for sodium analysis in blood. • Direct potentiometry is more accurate. What is New: • Less than 30% of pediatric articles provide information on the technique employed for sodium analysis in blood. • Indirect potentiometry is more frequently employed than direct potentiometry in pediatric studies.