Laboratory diagnosis of G6PD deficiency: BSH Guideline

 British Society for Haematology has released its guidelines on laboratory diagnosis of G6PD deficiency which is an update of the first G6PD guideline. The guidelines have been published in the British Journal of Hematology.

Glucose‐6‐phosphate dehydrogenase (G6PD) is a housekeeping enzyme expressed in all tissue cells where it catalyses the first step in the pentose phosphate pathway. In the red blood cell, this is the sole pathway for the production of NADPH, which is required to maintain glutathione in the reduced state (GSH). 

Following are the major recommendations: 

• Whole blood samples should not be stored for more than five days if anticoagulated with EDTA or not for more than three weeks if anticoagulated with ACD (one week for the cytochemical test) (GRADE 1C).
  
• Check the absorbance of the spectrophotometer for NADPH at the bandwidth (slot width) used and use the value obtained in your calculations. The theoretical molar extinction (of 6·22) is only obtained with narrow (≤4 nm) bandwidth instruments that are regularly serviced (GRADE 2C).
  
• Do not rely on screening tests for female patients; measure G6PD activity by quantitative spectrophotometric assay directly (GRADE 1C).
  
• The quantitative assay must be carried out if the screening test is abnormal or borderline (GRADE 1C).
• Re‐assay following a hemolytic episode of unknown cause to ensure the diagnosis of G6PD deficiency is not missed (GRADE 1C).
• Since the G6PD reaction is temperature‐dependent, an accurate cuvette temperature is very important (GRADE 1C):
• Where possible, measure the temperature inside the reaction cuvette using a certificated thermometer/thermistor. Use a thermostatically controlled, recirculating water bath or a Peltier heating component attached to the spectrophotometer or use an automated analyzer for the assay.
• Run assays at either 30°C or 37°C. In laboratories without air‐conditioning, a temperature of 37°C may be easier to control than 30°C unless a cooling unit is available (and these are expensive).
• Allow the assay mixture or reagents to equilibrate at the assay temperature before starting measurements.
• Report results at the temperature assayed, rather than using a "correction factor" to adjust the numeric results to those that might have been achieved at a different temperature.
• Run controls with every batch of samples: a normal and deficient sample obtained through a commercial company is preferable to an in‐house control (GRADE 1C).

This guideline is an update of the first G6PD guideline [The Assessment of Glucose‐6‐Phosphate Dehydrogenase Deficiency; prepared by the General Haematology Task Force, 1991]. Data from recent External Quality Assessment (EQA) exercises show that there is continued variation in both the results obtained and laboratory practice and this may be sufficient to affect clinical outcomes. The guideline is for use by staff working in diagnostic laboratories and is intended to promote the harmonization of analytical methods through sharing best practice in the diagnosis of G6PD deficiency.

• White cells contain a significant amount of G6PD, and ideally, they should be removed prior to assay, and especially if the count is above the lab's reference interval upper limit consider removal of white cells with a cellulose/"real" cotton wool column before assay (see Beutler, 1984) (GRADE 1C).

• Perform assays in duplicate if assays are undertaken only infrequently (GRADE 1C). On normal samples, the results of these duplicates should be within 0·5 iu/g of hemoglobin of each other (GRADE 1C).
• Establish a laboratory reference range by in‐house testing; referring to a kit manufacturer's leaflet (if using a commercial kit) or literature search only as a guide (GRADE 1C).
• Check that the assay absorbance increases in a linear fashion (it may take a minute or two for this to be achieved) and measure the absorbance over 10 min at 20 s intervals (for non‐kit methods) (GRADE 1C).
• Measuring the hemoglobin concentration of the hemolysate is just as important as measuring the enzyme activity, as both measurements will affect the final result to a similar extent. Similarly with well mixed whole blood where a kit indicates such use (GRADE 1C)
• The final G6PD activity should be interpreted in light of the reticulocyte count measured on the same sample (GRADE 1C).
  
• The MCH (pg) of the test sample should be taken into account, as very low values (as seen in thalassemia and iron deficiency) will overestimate the G6PD level where results are expressed in units per g hemoglobin (GRADE 1D).
• Molecular analysis can be undertaken if results of initial diagnostic procedures are equivocal or borderline, especially in (heterozygous) women and male individuals with Klinefelter syndrome (XXY) who may have intermediate levels similar to those of heterozygous females (GRADE 1D).
  
• Laboratories undertaking these screening tests and assays should participate in an accredited External Quality Assessment Schemes (GRADE 1C).

For more details click on the link: https://doi.org/10.1111/bjh.16366