Electrolytes
Sodium
Sodium is the principal cation in extracellular fluid and the main determinant of plasma osmolality and water balance. At the point of care it is measured rapidly by ion-selective electrode (ISE), usually as part of a combined electrolyte or blood gas panel.
Why it is measured
Sodium results help assess fluid status, dehydration and disorders of water handling, and abnormal values (hyponatraemia or hypernatraemia) are common reasons for urgent clinical review. Rapid availability at or near the patient supports critical care, emergency and remote settings where timely electrolyte data informs care.
| Typical range | Indicative adult range approximately 135 to 145 mmol/L (equivalent to mEq/L). Ranges vary by method and analyser, particularly between direct ISE on whole blood and indirect ISE on diluted serum or plasma, so always apply the range supplied by the reporting device or laboratory. |
|---|---|
| Sample | Whole blood (arterial, venous or capillary) at the point of care, or serum or plasma in the central laboratory. Measured by ion-selective electrode. Cartridge based handhelds typically need only two to three drops of blood. |
| Turnaround | Around 2 minutes on cartridge based handheld analysers, and broadly under 5 minutes on most POCT blood gas and electrolyte platforms. |
Point of care devices that report it
- Abbott i-STAT 1 (CHEM8+, EC8+, EG7+ and CG8+ cartridges)
- Siemens Healthineers epoc Blood Analysis System
- Radiometer ABL90 FLEX blood gas analyser
- Nova Biomedical Stat Profile Prime
- Roche cobas b 221 blood gas analyser
Questions, answered
Why might a point-of-care sodium differ from the central laboratory value?
Most handheld POCT devices use direct ISE on undiluted whole blood, whereas many central analysers use indirect ISE on diluted serum or plasma. The two methods can disagree, especially when protein or lipid levels are abnormal, where the indirect method may read falsely low (pseudohyponatraemia). Small differences of a few mmol/L are expected, so results should be interpreted against the range supplied for that specific method rather than transferred between platforms.
What sample types are accepted for POCT sodium?
Cartridge based analysers typically accept arterial, venous or capillary whole blood, often only two to three drops. Avoid sampling from a line that is running sodium containing fluids, and analyse promptly. Follow the manufacturer's instructions for the correct anticoagulant, since several platforms require balanced (lithium or sodium) heparin.
What are the critical alert thresholds for sodium?
Many services set critical (panic) limits at roughly 120 mmol/L at the low end and 160 mmol/L at the high end, but exact thresholds are defined locally. Point-of-care programmes should agree critical result flags and an escalation pathway with the laboratory so that extreme values trigger prompt review. This is operational guidance and not a substitute for local policy.