Standard Operating Procedure: G6PD Quantitative Assay

Introduction

This Standard Operating Procedure (SOP) outlines the standardized clinical laboratory protocol for the quantitative determination of Glucose-6-Phosphate Dehydrogenase (G6PD) activity in human whole blood. G6PD deficiency is a common hereditary enzymopathy that can lead to acute hemolysis upon exposure to oxidative stressors. This procedure is designed to ensure analytical accuracy, minimize pre-analytical errors—such as sample degradation—and ensure that results are reported in adherence to clinical diagnostic quality standards.

1. Pre-Analytical Requirements and Preparation

• Specimen Collection: Obtain 2–3 mL of whole blood via venipuncture into a Lavender-top (EDTA) or Green-top (Heparin) tube.
• Verification: Confirm patient identity using two unique identifiers and ensure the requisition form specifies "Quantitative G6PD."
• Sample Integrity: Inspect the specimen for hemolysis or clotting; reject if the sample is clotted or has been stored improperly.
• Timing: If testing cannot be performed within 4 hours, store the specimen at 2–8°C. Do not freeze whole blood.
• Documentation: Record the collection time and time of receipt in the Laboratory Information System (LIS).

2. Reagent and Instrument Setup

• Calibration: Verify that the spectrophotometer/analyzer has passed daily Quality Control (QC) and is calibrated according to manufacturer specifications.
• Reagent Inspection: Check the expiration date of the G6PD assay kit. Ensure reagents have been brought to room temperature (18–25°C) if stored at 2–8°C.
• Baseline Check: Run a reagent blank (using deionized water or saline) to ensure the optical path is clear and stable.

3. Execution of the Assay

• Sample Preparation: Thoroughly mix the whole blood specimen via gentle inversion.
• Hemolysate Preparation: Prepare the sample hemolysate by mixing the whole blood with the designated lysing reagent provided in the kit. Follow the precise dilution ratio mandated by the specific assay manufacturer.
• Centrifugation: Centrifuge the mixture at the recommended speed (e.g., 3000 rpm) for the required duration to remove cell debris.
• Reaction Initiation: Pipette the required volume of hemolysate and substrate/coenzyme mix into the cuvette or reaction well.
• Measurement: Initiate the kinetic reading at 340 nm. Record the change in absorbance ($\Delta A$) per minute over the programmed timeframe (usually 3–5 minutes).
• Normalization: Simultaneously measure the hemoglobin concentration of the sample to normalize the G6PD activity (expressed as U/g Hb).

4. Post-Analytical and Reporting

• Calculation: Ensure the LIS or automated software applies the correct formula: $G6PD (U/g Hb) = (\Delta A/min \times Total Volume \times 10^6) / (6.22 \times Sample Volume \times Hb concentration)$.
• Validation: Review the reaction curve for linearity. If the curve is non-linear, investigate for substrate exhaustion or reagent failure.
• Reporting: Compare results against the laboratory’s age/sex-specific reference intervals. Flag abnormal results for pathologist review if required by internal policy.

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Pro Tips & Pitfalls

• Pitfall - Recent Hemolysis: Do not perform this test during or immediately following an acute hemolytic episode. High reticulocyte counts in young, active red blood cells may produce "false normal" G6PD results, masking the underlying deficiency.
• Pitfall - Improper Storage: Never freeze whole blood for G6PD testing, as this will lead to enzyme denaturation and inaccurate, low results.
• Pro Tip - Precision Pipetting: Since the G6PD reaction is highly sensitive, ensure absolute precision in the hemolysate-to-reagent ratio. Use calibrated micropipettes.
• Pro Tip - Baseline Monitoring: If results are unexpectedly low, check the reagent blank. If the blank absorbance is high, the substrate may be degrading.

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Frequently Asked Questions (FAQ)

Q: How long can a blood sample be kept before the G6PD test is compromised? A: When stored at 2–8°C, a whole blood specimen in EDTA is generally stable for up to 5–7 days. However, testing should be performed as soon as possible to ensure maximum enzyme stability.

Q: Why is hemoglobin normalization required? A: G6PD activity is dependent on the number of red blood cells present. By normalizing the result to the hemoglobin concentration (U/g Hb), we eliminate variability caused by differences in the patient's hematocrit levels.

Q: What should I do if the result is in the "intermediate" range? A: Intermediate results may indicate a heterozygous female (mosaicism due to X-inactivation). In such cases, it is recommended to repeat the test, consider genetic counseling, or suggest molecular testing (DNA analysis) for definitive diagnosis.