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HelpTest Code GPI1 Glucose Phosphate Isomerase Enzyme Activity, Blood
Specimen Required
Container/Tube:
Preferred: Yellow top (ACD solution B)
Acceptable: Lavender top (EDTA)
Specimen Volume: 6 mL
Collection Instructions: Send whole blood in original tube. Do not aliquot.
Useful For
Evaluating individuals with Coombs-negative chronic hemolysis
Method Name
Kinetic Spectrophotometry (KS)
Reporting Name
Glucose Phosphate Isomerase, BSpecimen Type
Whole Blood ACD-BSpecimen Minimum Volume
1 mL
Specimen Stability Information
| Specimen Type | Temperature | Time |
|---|---|---|
| Whole Blood ACD-B | Refrigerated | 20 days |
Reject Due To
| Gross hemolysis | Reject |
| Fully clotted | Reject |
Clinical Information
The glucose 6-phosphate (G6P) isomerase enzyme interconverts G6P and fructose-6-phosphate in the second step of glycolysis. Glucose phosphate isomerase (GPI) deficiency (OMIM 613470) is a cause of nonspherocytic hemolytic anemia and has been reported in patients from varied ethnic backgrounds. As investigational methods have improved, the number of confirmed diagnoses has increased, although the disorder remains rare. Inheritance is autosomal recessive. Clinically significant GPI deficiency manifests in variable severity ranging from mild to severe anemia, with jaundice, gallstones, splenomegaly. Some cases of neonatal death/hydrops fetalis have been reported to be associated with GPI deficiency. A subset of patients shows neurologic impairment and granulocyte dysfunction. Heterozygotes are expected to have a normal phenotype.
Reference Values
≥12 months: 40.0-58.0 U/g Hb
Reference values have not been established for patients younger than 12 months.
Interpretation
Most clinically significant hemolytic anemias due to glucose phosphate isomerase (GPI) deficiency are associated with activity levels under 30% of mean normal; however, some clinically affected patients can have higher activity due to reticulocytosis. Heterozygous individuals usually show 40% to 60% of mean normal activity and are hematologically normal.
Variability of increased GPI activity is seen when young red blood cells are produced in response to anemia (reticulocytosis) or in newborns.
Cautions
Recent transfusion may mask the patient's intrinsic enzyme activity and cause unreliable results.
Reticulocytosis can mask some glucose phosphate isomerase deficiency cases by raising the activity level. Comparison to other red blood cells enzyme activity levels or correction for reticulocytosis may be useful.
Method Description
Glucose phosphate isomerase (GPI) interconverts glucose 6-phosphate (G6P) and fructose 6-phosphate (F6P). In this assay, the F6P is then further converted to 6-phosphogluconate (6-PG) through the G6P dehydrogenase (G6PD) reaction resulting in the reduction of nicotinamide adenine dinucleotide phosphate (NADP[+]) to NADPH. The reduction of NADP(+) is measured spectrophotometrically by the increase in absorbance at 340 nm on an automated chemistry analyzer.(Beutler E. Red Cell Metabolism: A Manual of Biochemical Methods. 3rd ed. Grune and Stratton; 1984:40-42; Rab MAE, van Wijk R. Enzymes of the red blood cell. In: Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023:chap 78)
Performing Laboratory
Mayo Clinic Laboratories in Rochester
Test Classification
This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. It has not been cleared or approved by the US Food and Drug Administration.CPT Code Information
84087
LOINC Code Information
| Test ID | Test Order Name | Order LOINC Value |
|---|---|---|
| GPI1 | Glucose Phosphate Isomerase, B | 44050-3 |
| Result ID | Test Result Name | Result LOINC Value |
|---|---|---|
| GPICL | Glucose Phosphate Isomerase, B | 44050-3 |
Day(s) Performed
Tuesday, Thursday