SOP-C-004 — Serum Electrolytes (Sodium, Potassium, and Chloride) Testing on Sensa Core ISE Electrolyte Analyzer SOP
Type: SOP · Version: V0 · Status: DRAFT — UNAPPROVED · Served read-only from the bundled QMS copies. The Drive master is untouched.
Related visual material
WI-C-004-A-V0 — Sensa Core ISE Electrolyte Analyzer Operation & Daily Cleaning Bench Card
Rendered in-browser from the project's Word file SOP-C-004-serum-electrolytes-na-k-cl-sensa-core-ise-analyzer-sop.docx (policies-docx / forms-docx — the file Junry reviews). Formatting is faithful, not pixel-perfect; the DOCX is the official copy.
1.0 Purpose
To establish a standardized technical operating procedure for the direct potentiometric measurement of serum Sodium (Na+), Potassium (K+), and Chloride (Cl-) using the Sensa Core Ion-Selective Electrode (ISE) Electrolyte Analyzer at Tarangnan Community Hospital.
2.0 Scope
This procedure applies to all Medical Technologists performing routine and emergency electrolyte analyses in the Clinical Chemistry section. It covers specimen collection criteria, prevention of pseudohyperkalemia, reagent pack management, two-point and drift calibrations, electrode cleaning cycles, and critical value communications.
3.0 Definitions
- Direct Potentiometry: An electrochemical analytical method that measures the electrical potential difference between an ion-selective electrode and a stable reference electrode in an undiluted sample, measuring free active ion concentration without volume displacement artifacts.
- Ion-Selective Electrode (ISE): A sensor with an ion-permeable membrane that generates an electrical potential proportional to the logarithm of the activity of the selected ion in solution.
- Nernst Equation: The fundamental thermodynamic equation relating electrical electrode potential (E) to ion activity (a): E = E0 + (2.303 RT / nF) log(a).
- Electrode Slope: The change in electrical potential (in millivolts, mV) per tenfold (one decade) change in ion activity; an essential indicator of electrode membrane health and response sensitivity.
- Pseudohyperkalemia: An in vitro artifactual elevation of measured serum potassium concentration caused by mechanical hemolysis, prolonged clot contact, fist pumping, or potassium-EDTA contamination in the absence of true physiological hyperkalemia.
4.0 Responsibility
4.1 Hospital Director
- Grants final authorization and approval for this standard operating procedure.
- Ensures administrative support and resource allocation for Sensa Core closed reagent packs, replacement electrodes, and external proficiency surveys.
4.2 Pathologist
- Conducts technical and clinical review of electrolyte measurement protocols.
- Provides consultative guidance on unexplained electrolyte shifts, acid-base discrepancies, and extreme critical values.
4.3 Chief Medical Technologist / Laboratory Quality Manager
- Monitors daily electrode slope parameters, two-point calibration logs, and two-level quality control performance.
- Directs electrode maintenance, deproteinization routines, and electrode replacements per Equipment Temperature Monitoring Policy.
- Oversees the investigation of pseudohyperkalemia occurrences in coordination with the nursing service and phlebotomy team.
4.4 Medical Technologists
- Inspect all electrolyte specimens for hemolysis, tube type, and clotting adequacy before presentation to the analyzer.
- Perform daily instrument startup, two-point slope calibration, and two-level quality control before testing patient samples.
- Execute daily end-of-shift enzyme cleaning and weekly sodium electrode conditioning.
- Validate analytical results, initiate prompt critical value telephone notifications, and log read-back records per Reporting Critical Values Policy.
4.5 Laboratory Aides
- Transport centrifuged, unhemolyzed serum tubes from the receiving station to the chemistry bench.
- Maintain work area cleanliness and ensure adequate supply of disposable sample cups and thermal printer paper under Medical Technologist direction.
- Shall not operate the Sensa Core analyzer, present samples to the probe, calibrate electrodes, or release electrolyte results.
5.0 Policy / Procedure
5.1 Analytical Principle and Sensor Technology
- Direct Potentiometry: - The Sensa Core ST-200 series electrolyte analyzer employs direct potentiometric measurement across a flow-through sensor block containing three maintenance-free parameter electrodes and one silver / silver chloride (Ag/AgCl) reference electrode: - Sodium Electrode: Solid-state glass capillary membrane responsive to sodium ions. - Potassium Electrode: Plasticized polyvinyl chloride (PVC) membrane impregnated with the neutral macrocyclic carrier valinomycin, providing high selectivity for potassium over sodium. - Chloride Electrode: Ion-exchanger polymeric membrane incorporating quaternary ammonium salts selective for chloride anions. - Reference Electrode: Ag/AgCl reference junction with a stable internal filling solution containing saturated potassium chloride (KCl).
- Measurement Mechanism: - When serum enters the measuring chamber, an electrical potential develops across each selective membrane governed by the Nernst relationship: E = E0 + S log(ai) where E is the measured electromotive force, E0 is the standard electrode potential, S is the operational slope (~59.16 mV/decade at 25 °C for monovalent cations), and ai is the active ion activity.
5.2 Strict Specimen Collection and Pseudohyperkalemia Prevention
- Acceptable Specimen Types: - Fresh non-hemolyzed Serum collected in plain Red-Top (clot activator) or Gold-Top (SST gel separator) tubes. - Lithium Heparin Plasma collected in green-top tubes. Lithium heparin is the only acceptable anticoagulant for plasma electrolyte determination.
- Prohibited Tubes and Rejection Criteria: - K2-EDTA or K3-EDTA Tubes (Lavender-Top): Prohibited. EDTA contains high concentrations of potassium salts (dipotassium or tripotassium EDTA). Drawing blood into or transferring from an EDTA tube causes catastrophic false hyperkalemia (> 15.0 mmol/L) accompanied by severe pseudo-hypocalcemia. - Sodium Citrate Tubes (Light Blue-Top): Prohibited due to sodium salt contamination and significant dilution bias. - Sodium Fluoride / Potassium Oxalate Tubes (Gray-Top): Prohibited due to direct sodium and potassium salt contamination.
- Prevention of Pre-Analytical Hemolysis: - Erythrocyte intracellular potassium concentration (~150 mmol/L) is approximately 25 times greater than extracellular serum concentration (~4.0 mmol/L). - Even trace hemolysis (free hemoglobin 50 mg/dL, causing faint pink discoloration) raises serum potassium by 0.3 to 0.5 mmol/L. - Visible hemolysis (free hemoglobin > 100 mg/dL, frank red serum) causes dangerous pseudohyperkalemia and SHALL BE REJECTED for potassium testing. Medical Technologists must request an immediate recollection.
- Phlebotomy and Handling Instructions: - Phlebotomists shall avoid prolonged tourniquet application (> 1 minute) and instruct patients not to clench or pump their fist repeatedly during venipuncture. - Centrifuge and separate serum from erythrocytes within 1 to 2 hours of collection. Prolonged contact at ambient temperature leads to erythrocyte adenosine triphosphate (ATP) depletion, disabling the Na+/K+ ATPase pump and causing massive potassium leakage into the serum.
| Specimen Parameter | Acceptable Specification | Unacceptable Condition / Bench Action |
|---|---|---|
| Primary Tube | Red-Top Clot Activator or Gold-Top SST | Lavender EDTA, Blue Citrate, Gray Fluoride (Reject immediately) |
| Clotting Time | 20–30 minutes upright at 20–25 °C | Centrifuging un-clotted blood (causes latent fibrin and probe clots) |
| Hemolysis Index | Clean, straw-colored serum (free Hgb < 50 mg/dL) | Visible pink/red serum (Reject potassium; request urgent recollection) |
| Separation Limit | Separate serum from cells within 1–2 hours | Specimen standing on clot > 2 hours (Reject for potassium) |
| Storage Stability | 4 hours at room temperature; 48 hours at 2–8 °C | Repeated freeze-thaw cycles (prohibited) |
5.3 Reagent Pack Verification and System Setup
- Closed Reagent Module: - The Sensa Core analyzer uses a snap-in closed reagent pack containing: - Calibrator A: Primary low standard with known electrolyte concentrations. - Calibrator B: High standard used for slope determination. - Waste Bag: Sealed internal waste compartment with check valve to eliminate biohazard exposure. - Inspect reagent pack expiration date and remaining percentage display on the instrument screen. Replace the pack when volume reaches < 5% or expiration date is reached.
- Peristaltic Pump and Fluidic Line Verification: - Visually inspect the peristaltic pump tubing and valve connections for wear, pinches, or crystalline deposits. - Ensure the reference electrode fluid line is primed and free of trapped air bubbles.
5.4 Calibration and Maintenance Sequences
- Automated Two-Point Calibration: - The analyzer automatically initiates a 2-point calibration (Calibrator A and Calibrator B) upon startup, every 4 to 8 hours during active operation, and after waking from standby. - The instrument aspirates Cal A, measures the baseline potential, aspirates Cal B, measures the secondary potential, and computes the slope: Slope (mV/decade) = (E_CalB - E_CalA) / log([Cal B] / [Cal A]) - Acceptable Slope Criteria: - Sodium (Na+): 50.0 to 62.0 mV/decade - Potassium (K+): 50.0 to 62.0 mV/decade - Chloride (Cl-): 40.0 to 58.0 mV/decade - If an electrode slope falls outside acceptable limits, the analyzer displays an error flag (such as Na Slope Low or K Unstable). Testing is automatically locked.
- Single-Point Drift Calibration: - The analyzer aspirates Calibrator A immediately before or after every sample run. - This single-point adjustment corrects for ambient temperature variations and minor electronic baseline drift.
- Daily Two-Level Quality Control (IQC): - Medical Technologists shall analyze Level 1 (Normal) and Level 2 (Pathological / Abnormal) aqueous or serum-matrix electrolyte controls each morning before testing patient specimens. - Document QC values in the Sensa Core Daily QC Logbook. Results must fall within established target ranges (± 2 SD). If QC fails, recalibrate the instrument and re-test before releasing patient results.
- Daily Cleaning and Deproteinization: - At the conclusion of each daily testing shift, perform automated daily cleaning. - Present the Sensa Core Daily Cleaning Solution (containing active proteolytic pepsin deproteinizer) to the sample probe. - The solution bathes the flow-through electrode channel for 3 minutes to remove adhering serum proteins from the PVC membranes, followed by an automated flush with Calibrator A.
- Weekly Sodium Electrode Conditioning: - Once weekly, aspirate the dedicated Sodium Conditioning Solution (ammonium bifluoride / electrolyte conditioner) into the electrode chamber. - This process revitalizes the hydrated gel layer of the sodium glass capillary membrane. Perform a 2-point calibration following conditioning.
5.5 Stepwise Patient Sample Analysis
- Ensure the analyzer displays READY and confirms acceptable 2-point calibration.
- Centrifuge patient serum at 1500–2000 × g for 10 minutes.
- Uncap the tube, verify patient identity, and inspect serum for hemolysis or fibrin clots.
- Lift the sample arm or press Analyze on the touch screen; present the serum tube to the aspiration probe.
- The analyzer automatically aspirates 65 to 100 µL of sample, withdraws the probe, and initiates measurement.
- Observe the display: within 45 to 60 seconds, measured concentrations for Na+, K+, and Cl- are displayed and printed.
- Recap the primary tube; verify that the analyzer executes its wash cycle and returns to READY.
5.6 Reference Intervals and Critical Values
- Approved TCH Electrolyte Reference Intervals:
| Electrolyte Parameter | Standard Units | Adult Reference Interval | Critical Low Threshold | Critical High Threshold |
|---|---|---|---|---|
| Serum Sodium (Na+) | mmol/L | 135 to 145 mmol/L | < 120 mmol/L | > 160 mmol/L |
| Serum Potassium (K+) | mmol/L | 3.5 to 5.1 mmol/L | < 2.8 mmol/L | > 6.2 mmol/L |
| Serum Chloride (Cl-) | mmol/L | 98 to 107 mmol/L | < 80 mmol/L | > 120 mmol/L |
- Critical Value Notification Protocol: - When an electrolyte result breaches critical thresholds: - Re-examine the specimen immediately on the Sensa Core analyzer to rule out technical or sampling artifact. - Inspect the tube for subtle hemolysis and verify that the collection tube was not drawn after an IV fluid infusion (such as normal saline or potassium chloride piggyback). - Telephone the attending physician or inpatient ward nurse within 15 minutes per Reporting Critical Values Policy. - Require mandatory verbal read-back and document all details in F-QM-POST-002-A-V0.
5.7 Troubleshooting and Operational Limitations
- Electrode Slope Failure: - If slope is low or drifting, execute two consecutive manual clean cycles with Daily Cleaning Solution. - If the condition persists, replace the affected individual electrode and perform full 2-point calibration.
- Liquid Flow Clots: - If the analyzer displays Sample Not Detected or Aspiration Error, remove the probe wash block and inspect the sample path for fibrin strands. Use the supplied stylus wire to clear the probe orifice gently.
- Severe Hyperlipidemia: - Direct potentiometry measures ion activity in the aqueous phase and is impervious to electrolyte exclusion effects caused by severe hypertriglyceridemia or paraproteinemia (unlike indirect chemistry methods). However, extreme milky turbidity may coat membrane surfaces; execute extra cleaning cycles following lipemic samples.
6.0 Records
| Record Description | Controlled Form / Log | Storage Location | Retention Authority |
|---|---|---|---|
| Sensa Core Daily Calibration and Slope Log | Electrolyte Workstation Log | Clinical Chemistry Bench | Retained per Laboratory Retention Schedule Policy |
| Electrolyte Daily Two-Level QC Log | Chemistry Section QC Binder | Quality Control Archive | Retained per Laboratory Retention Schedule Policy |
| Electrolyte Patient Test Printouts and Worksheets | Daily Chemistry Files | Laboratory Records Archive | Retained per Laboratory Retention Schedule Policy |
| Critical Value Notification Records | F-QM-POST-002-A-V0 | Critical Value Logbook | Retained per Laboratory Retention Schedule Policy |
| Sensa Core Electrolyte Analyzer Operation Bench Aid | A V0 | Posted at Sensa Core ISE Analyzer | Retained per Laboratory Retention Schedule Policy |
7.0 References
- DOH Clinical Laboratory QMS Tool, Section 3 — Service Delivery
- DOH Administrative Order No. 2021-0037, Annex B1 — Technical Standards for Clinical Laboratories
- ISO 15189:2022, Clause 7.3 — Examination Processes
- WHO LQMS Handbook, Chapter 7 — Quality Control for Clinical Chemistry
- Sensa Core ST-200 Series ISE Electrolyte Analyzer Operator's Manual (Sensa Core Medical Instrumentation Pvt. Ltd.)
Companion policies:
- Clinical Chemistry Section Workflow Quality Control and Validation Sop — Clinical Chemistry Section workflow, quality control, and result validation
- Reporting Critical Values Policy — Immediate critical value notification, documentation, and read-back verification
- Laboratory Retention Schedule Policy — Retention periods and disposition schedule for clinical chemistry records