Introduction Hyponatremia is a common electrolyte disturbance in preterm neonates due to immature renal function and fluid management challenges. Accurate and timely assessment of serum sodium is critical in this vulnerable population. This study aimed to determine the incidence of hyponatremia in preterm neonates on intravenous fluids and to compare sodium levels measured by blood gas analyzers (BGA) and laboratory autoanalyzers in a tertiary care NICU in North India. Methods A prospective observational study was conducted over six months in a Level III NICU. Thirty preterm neonates receiving >
50% of their total fluids via IV by Day 7 were enrolled. Venous blood samples were collected on Days 3, 5, and 7 and analyzed for sodium using both BGA (direct ion-selective electrode (ISE)) and laboratory autoanalyzer (indirect ISE). Hyponatremia was defined as sodium <
135 mmol/L. Agreement between methods was evaluated using Bland-Altman plots and Deming regression. Results Hyponatremia incidence peaked on Day 5 (26.7% via BGA), with 23.3% on Day 3 and 10.0% on Day 7. Laboratory analysis showed similar trends. Bland-Altman plots and regression analysis demonstrated strong agreement between BGA and lab measurements, with differences remaining within clinically acceptable limits. Higher rates of hyponatremia were observed in neonates with respiratory distress syndrome and sepsis. Conclusion Hyponatremia remains a prevalent concern in preterm neonates, particularly among those with comorbidities. BGA is a reliable point-of-care tool for sodium monitoring when venous samples are used. These findings call for a re-evaluation of current fluid and electrolyte administration strategies during the first week of life to better address the unique physiological needs of preterm infants.