Introduction
The principal extracellular cation in the extracellular fluid, sodium, is essential for the preservation of tissue hydration, acid-base balance, and osmotic pressure. Serum sodium levels are crucial markers in critical care management that direct clinical choices for patient care. It is crucial to measure salt levels accurately. Newer, more effective techniques for calculating sodium levels in blood and serum have been developed over time.1
The creation of salt with zinc uranyl acetate and subsequent gravimetric, titrimetric, or colorimetric quantification were early procedures for determining serum sodium levels. To measure low sodium concentrations in solution, Barnes, Richardson, Berry & Hood later developed the flame photometer. R.W. Bunsen, Gustav, and R. Kirchhoff developed atomic absorption spectrometry, which was used to calculate the electrolytes present in various bodily fluids. However, these procedures frequently required lengthy turnaround times, had little sample throughput, and were difficult to use.2
The Ion Selective Electrode (ISE) evolved as the current standard reference method for quick and accurate monitoring of sodium levels with the development of improved techniques. Numerous research has examined various electrolyte estimate techniques. One study established ISE as the reference method by showing that the colorimetric approach had an adequate analytical performance for sodium and potassium while flame photometry did not fulfill the criteria. These investigations examined the sodium readings from the flame photometer with both the direct and indirect ISE methods, finding a good agreement between the two and arguing that they may be used interchangeably. Comparing electrolyte measurements made with an auto-analyzer and arterial blood gas equipment, it was also suggested to use caution when comparing these measurements.3
The Ion Selective Electrode (ISE) evolved as the current standard reference method for quick and accurate monitoring of sodium levels with the development of improved techniques. Numerous research has examined various electrolyte estimate techniques. One study established ISE as the reference method by showing that the colorimetric approach had an adequate analytical performance for sodium and potassium while flame photometry did not fulfill the criteria. These investigations examined the sodium readings from the flame photometer with both the direct and indirect ISE methods, finding a good agreement between the two and arguing that they may be used interchangeably. Comparing electrolyte measurements made with an auto-analyzer and arterial blood gas equipment, it was also suggested to use caution when comparing these measurements.4
Material and Methods
Inclusion & exclusion criteria
The study comprised 120 serum samples from patients with ages ranging from 18 to 80, of either gender, that were given to the lab with a request for electrolyte calculation. Excluded samples included those that were haemolyzed, turbid, lipemic, or hyperbilirubinemia.
Ethical approval & informed consent
Every patient or his family member who requested electrolytes was asked to sign a written informed consent form after the Ethics Committee of the Medical College granted its approval. (Enclosed).
Methods
Indirect ISE on the Roche Cobas c501 chemistry analyzer (Roche Diagnostics GmbH, Mannheim, Germany) or direct ISE on the Roche AVL 9180 (9180) electrolyte analyzer (Roche Diagnostics) were used to analyze the samples that were collected in response to a request for electrolytes. The same serum sample was subsequently used to detect sodium using a sodium electrolyte colorimetric test kit within two hours.
Results
A total of 120 serum samples were collected through simple random sampling and analyzed for sodium (Na+) using two different instruments: Roche Cobas (c501) and Roche AVL 9180 (9180) by Indirect ISE and Direct ISE methods, respectively. Statistical analysis was conducted using SPSS, NCSS, and MINITAB software, with a significance level of p<0.05. Descriptive statistics, correlation analysis, scatter plots, and linear regression were performed to assess the relationship between the instruments. The comparison of sodium values was primarily focused on the hyponatremic and normonatremic ranges due to limited samples in the hypernatremia range.
Table 1
Classification of sodium (Na+) levels and reference ranges in different groups
|
S. No. |
Analyte |
Reference Ranges (mmol/L) |
Groups |
|
1. |
Sodium (Na+) |
134 – 147 |
Normonatremia |
|
2. |
Sodium (Na+) |
<134 |
Hyponatremia |
|
3. |
Sodium (Na+) |
>147 |
Hypernatremia |
The sodium (Na+) values obtained from the C501 instrument and the 9180 instruments were further classified into three groups based on the reference ranges (Table 1).
Table 2
Mean, S.D and CV% for sodium levels between C501, 9180 and colorimetry
|
|
N |
Mean |
Std. Deviation |
CV% |
|
Na+ c501 |
100 |
134.32 |
11.21 |
8.57 |
|
Na+ Colorimetry |
100 |
145.89 |
17.96 |
11.41 |
|
Na+ 9180 |
35 |
132.01 |
9.79 |
6.64 |
|
Na+ Colorimetry |
35 |
139.13 |
15.27 |
12.04 |
Descriptive statistics, including the mean, standard deviation (S.D), and coefficient of variation (CV), were calculated for the sodium values (Table 2).
Table 3
Pearson’s correlation between Na+ c501 vs colorimetry & Na+ 9180 vs colorimetry
|
|
N |
Na+ on Colorimetry |
P value |
|
Na+ on c501 |
100 |
0.495 |
<0.001 |
|
Na+ on 9180 |
35 |
0.487 |
<0.001 |
Table 4
Simple linear regression between instruments and ANOVA for unstandardized predicted values
Table 5
Na+ levels between the instruments in hyponatremia & normonatremia
Correlation analysis was performed using Pearson's correlation coefficient for the sodium values obtained from different instruments (Table 3). Scatter plots were also generated to visualize the correlation.
A simple linear regression analysis was conducted to establish a linear regression equation between the sodium values obtained from the C501 instrument and colorimetry, as well as between the sodium values obtained from the 9180 instruments and colorimetry. This analysis aimed to determine good fit.
A comparison of sodium values was performed only within the hyponatremic and normonatremic ranges (Table 5) due to the study's sampling method, which involved simple random sampling. Reference ranges for serum sodium were used to determine the classification of the samples.
Discussion
A vital analyte frequently measured in intensive care units and annual physicals are serum sodium. Although ion-selective electrodes (ISEs) are frequently employed in tertiary care institutions for salt estimation, they are expensive and logistically impractical for basic health centers, which could result in the underdiagnosis of hyponatremia in rural regions. To solve this problem, this study compared serum sodium measurements made with the colorimeter, a low-cost tool, to those made using ISEs.5
It was determined that a sample size of 10 would be adequate for statistical significance and a power of 99% based on a prior study comparing various sodium measurement techniques. However, greater sample size was suggested to account for manual mistakes related to the colorimetric approach and to attain desirable precision amongst equipment.6
Table 3 shows a moderately positive linear connection for sodium readings between the C501 and the colorimeter as well as between the 9180 and the colorimeter. To evaluate the comparability of sodium readings across the methods/instruments (indirect ISE/C501 vs.colorimetry and direct ISE/9180 vs. colorimetry), a straightforward linear regression analysis was conducted.7 The sodium levels were similar between the instruments, according to the ANOVA for unstandardized predicted values (Table 4), and the regression models offered a reasonable fit for the data. Significant F values (169.429 and 42.85, both with P<0.001) were shown by the ANOVA for unstandardized predicted values. The sodium levels on the C501 instrument and the 9180 instruments may be predicted based on the findings of the colorimetry using the regression equations created using the R2 value.8
The mean difference in sodium readings surpassed the proposed target value by 4 mmol/L, according to Bland-Altman analysis (Table 5).9 As a result, additional research was done primarily in the hyponatremic and normonatremic areas. In the hyponatremic range, the instruments had a significant positive connection, and in the normonatremic range, they had a moderately positive association. The prediction of sodium values on the C501 instrument and the 9180 instruments using the regression equations indicated better prediction accuracy in the hyponatremic range. The bland-Altman analysis confirmed that the mean difference for sodium levels fell within the suggested ±4 mmol/L range.10
Early-stage hypo and hypernatremia can be asymptomatic and show symptoms that resemble conditions of the central nervous system, including irritability, nausea, weakness, confusion, and seizures.11 Dysnatremia must be identified early since it raises the risk of mortality and morbidity. A low-cost tool like the colorimeter can be used in rural healthcare settings to help detect hyponatremia in its early stages, allowing for early intervention and enhancing the quality of life.
Limitations of our Study
It is well-established that quantities of glucose, protein, and lipids influence serum sodium levels. A significant flaw in our study is the inability to estimate the aforementioned components. Except for roughly 15 samples, the same sample was not analyzed for sodium on both the c501 and the 9180 because it is well-known that the results of direct and indirect ISE are equivalent. A bigger sample size estimate would have allowed for the evaluation of the Sodium colorimetric kit's sensitivity and specificity. Due to random sampling, the study also lacked a statistically significant number of hypernatremic samples.
Merits of the Study
This pilot project aims to highlight the value of a low-cost, straightforward tool for diagnosing hyponatremia in rural healthcare facilities, enabling prompt medical intervention and reducing the incidence of morbidity and mortality that would otherwise be associated with it.
Conclusion
The sodium values calculated by the Direct & Indirect ISE instruments are comparable to those determined by a colorimeter in the hyponatremic and normonatremic ranges. When the samples are analyzed using a colorimeter, the regression equations were developed to forecast the Na+ values on C501 and 9180. Using a low-cost colorimeter to screen for a curable ailment like hyponatremia in rural regions will give patients the chance to receive early interventional therapy, especially in the geriatric age groups.
