The Glucose-6-Phosphate Dehydrogenase (G6PD) assay is a specialized blood test that measures the activity level of the G6PD enzyme within red blood cells. This enzyme protects these cells from damage caused by certain chemicals and stresses. A deficiency is a common inherited disorder that can lead to the rapid destruction of red blood cells, known as hemolytic anemia. The assay is an important tool used by healthcare providers to screen for or confirm this deficiency, allowing for proactive management and the prevention of severe health complications.
Understanding G6PD Deficiency
The G6PD enzyme is a component of the pentose phosphate pathway in red blood cells. Its main function is to produce Nicotinamide Adenine Dinucleotide Phosphate (NADPH). NADPH is necessary to maintain a steady supply of reduced glutathione, the cell’s primary defense against oxidative stress.
Red blood cells are susceptible to damage because they are constantly exposed to oxygen and lack the machinery to produce new protective proteins as they age. When a person has a G6PD deficiency, the red blood cells cannot generate enough NADPH to neutralize harmful reactive oxygen species (free radicals). Exposure to certain substances can thus cause severe oxidative damage to the red blood cells.
This oxidative damage rapidly destroys the red blood cells, leading to acute hemolytic anemia. The genetic alteration is located on the X chromosome, making it an X-linked inherited condition. Consequently, it affects males more frequently and severely than females, although females can also be affected or be carriers.
Indications for Testing and Assay Types
The G6PD assay is ordered for several reasons, most frequently for newborn screening in high-prevalence populations. It is also used for adults and children experiencing unexplained hemolytic anemia, which may present with jaundice, fatigue, and dark urine. Testing is also advised before prescribing specific medications known to be oxidative stressors, such as certain antimalarial drugs.
Two main types of assays detect G6PD deficiency: qualitative and quantitative tests. Qualitative assays, like the fluorescent spot test, are used for rapid screening to determine if the enzyme is present or absent. While quick and straightforward, a qualitative test does not provide the precise level of enzyme activity.
The quantitative assay, often performed using spectrophotometric analysis, is the definitive test for diagnosis. This method accurately measures the exact amount of G6PD enzyme activity in the red blood cells. Quantitative tests are necessary to confirm an abnormal or uncertain result from a screening test, especially in females who may have intermediate enzyme levels.
Conducting the Assay and Interpreting Results
The G6PD assay is performed on a blood sample, usually collected from a vein in the arm or via a heel prick for newborns. In the laboratory, specialized equipment measures the rate at which the G6PD enzyme converts a substrate to a product, which is proportional to its activity. Results are reported as units of enzyme activity per gram of hemoglobin (U/g Hb) or as a percentage of the lab’s normal reference range.
A result indicating severe deficiency is less than 10% of the normal enzyme activity level. Moderate deficiency is classified as activity between 10% and 60% of the normal range, which can cause intermittent hemolysis upon exposure to triggers. A normal result means the enzyme activity is within the expected range, suggesting the patient does not have the deficiency.
A complicating factor is the possibility of a false-negative result if the test is performed during or immediately after an acute hemolytic episode. During a crisis, the older, most deficient red blood cells are destroyed and cleared. The remaining red blood cells are younger, possessing higher-than-usual enzyme activity. If G6PD deficiency is suspected despite a normal result during an acute event, the test should be repeated approximately three months later, once the red blood cell population has normalized.
Managing Life with G6PD Deficiency
Management of confirmed G6PD deficiency centers on preventing hemolytic episodes by avoiding oxidative triggers. Since the deficiency is a lifelong genetic condition, there is no cure, but it is managed effectively through preventative measures. The most well-known trigger is the fava bean, which causes a reaction known as favism in susceptible individuals.
Medical triggers include specific medications, such as certain antimalarial drugs like primaquine, some sulfa drugs, and common antibiotics. Infections, whether bacterial or viral, also create oxidative stress and are a common trigger for hemolysis. Healthcare providers maintain lists of known oxidative agents to guide prescribing decisions for affected patients.
Individuals with G6PD deficiency should inform all healthcare professionals, including pharmacists and dentists, before receiving any new prescription or over-the-counter medication. Carrying a medical identification card or wearing a medical alert bracelet is a practical safety measure in emergency situations. With careful avoidance of triggers, most people with G6PD deficiency can live a full and healthy life without complications.