Standard Operating Procedure: Serum Electrolytes (Na, K, Cl)

1. Purpose and Scope

To measure serum sodium, potassium, and chloride using ion-selective electrodes (ISE) for the assessment of fluid, electrolyte, and acid-base balance, and for the calculation of the anion gap.

2. Specimen Requirements

• Type: Serum, plasma (lithium heparin), or whole blood.
• Volume: Minimum 0.5 mL serum or 1.0 mL blood.
• Hemolysis: Avoid hemolysis; RBC rupture releases intracellular potassium, falsely elevating K+.
• Stability: Separate serum from cells within 2 hours (critical for potassium); store capped at 2-8°C and analyze within 24 h.
• Timing: Random sampling is acceptable; note hemolysis, lipemia, and icterus indices on the report.

3. Safety and Precautions

• Standard PPE (gloves, lab coat, eye protection). All blood samples are potentially infectious.
• Dispose of sharps and biohazard waste per laboratory policy.
• Do not pipette by mouth. Use mechanical pipetting devices.

4. Equipment and Reagents

• Automated chemistry analyzer with an ISE module (indirect or direct measurement).
• Sodium-selective, potassium-selective (valinomycin), and chloride-selective electrodes.
• Internal reference electrode and reference/salt-bridge solution.
• High-ionic-strength diluent buffer (for indirect ISE).
• Two levels of quality control material (normal and abnormal).

5. Per-Analyte Procedures

5.1 Sodium (Na+) Procedure - Indirect ISE

1. Prime the ISE module and flush lines with internal reference solution; discard the first aliquot if air bubbles are present.
2. Calibrate with two level standards (low and high) and verify electrode slope against the Nernst equation.
3. Auto-dilute serum 1:30 in high-ionic-strength buffer to minimize protein and lipid matrix effects.
4. Pass the diluted sample over the sodium-selective electrode; record the potential difference against the reference electrode.
5. Derive the Na+ concentration in mEq/L using the calibration curve; run two levels of control and release after acceptance.

5.2 Potassium (K+) Procedure - Indirect ISE

1. Calibrate the potassium electrode (valinomycin membrane) with low and high standards.
2. Auto-dilute serum 1:30 in buffer; pass over the potassium-selective membrane.
3. Record the potential difference against the reference electrode; convert to mEq/L by the Nernst equation.
4. Repeat critical values (> 6.0 or < 2.5 mEq/L) on a fresh, non-hemolyzed specimen before reporting.
5. Run two levels of control; reject samples flagged as hemolyzed.

5.3 Chloride (Cl-) Procedure - Indirect ISE

1. Calibrate the chloride electrode with low and high ion standards.
2. Auto-dilute serum 1:30 in buffer; pass over the chloride-selective membrane.
3. Record the potential against the reference electrode; convert to mEq/L.
4. Correlate chloride with sodium and CO2/bicarbonate for anion-gap interpretation.
5. Run controls; flag samples with abnormal hemolysis, lipemia, or icterus indices.

6. Anion Gap (derived)

• Anion gap = Na+ - (Cl- + HCO3-). Normal reference interval is approximately 8-16 mEq/L (non-Farr method), stated as a decision threshold in prose; verify the laboratory-specific interval.
• A high anion-gap metabolic acidosis (e.g., lactic acidosis, ketoacidosis, uremia, toxin ingestion) is suggested when the gap is elevated; a normal-gap (hyperchloremic) acidosis is suggested by a low bicarbonate with a normal gap.

7. Quality Control

• Run two levels (normal and abnormal) of control daily, with every calibration, and with each new electrode or reagent lot.
• Enforce Levey-Jennings rules; investigate trends, shifts, or electrode drift before reporting patient results.
• Perform electrode conditioning and maintenance (flushing, membrane care) per the manufacturer schedule.
• Participate in an external quality assessment (EQA) program for all three electrolytes.

8. Decision limits

Decision thresholds for critical electrolyte values are stated as prose; the canonical adult reference intervals are shown in the Reference Ranges panel.

• Critical hypokalemia: K+ < 2.5 mEq/L; critical hyperkalemia: K+ > 6.5 mEq/L - both require urgent clinical notification.
• Critical hyponatremia: Na+ < 120 mEq/L; critical hypernatremia: Na+ > 160 mEq/L - both require urgent clinical notification.
• Chloride: a disproportionate fall with a normal gap suggests a hyperchloremic (normal anion gap) acidosis.

Note on pseudohyponatremia: with indirect (dilutional) ISE, marked hyperproteinemia or hyperlipidemia can lower measured sodium (pseudohyponatremia); direct ISE on whole blood avoids this effect.