Diagnostic testing for gestational diabetes mellitus during the COVID-19 pandemic: an opportunity to revisit Glucose-Based testing.

Advice on diagnostic testing for gestational diabetes mellitus (GDM) during the COVID‐19 pandemic was recently provided by the Australasian Diabetes in Pregnancy Society and the Australian Diabetes Society. For women at low risk for GDM, an alternative method of testing for GDM involves an initial fasting blood glucose (FBG) and subsequent oral glucose tolerance test (OGTT) (75 g oral glucose load) for women with FBG 4.7–5.0 mmol/L; FBG of >5.0 mmol/L is diagnostic of GDM. OGTT is not indicated if FBG is <4.7 mmol/L.

The pre‐analytical and analytical variations for glucose‐based testing should be considered. A major source of pre‐analytical error is loss of glucose from blood specimens through glycolysis. Glucose is lost from whole blood samples at a rate of 5–7% per hour at room temperature. Pre‐analytical loss of glucose poses a threat to the diagnostic sensitivity of glucose‐based testing for GDM. The most commonly used blood collection tube for glucose is sodium‐fluoride‐based and it is widely used to inhibit glycolysis but it is inadequate. Sodium‐fluoride does not stop glycolysis for the first 2 h or more after sample collection, and during the first 60–90 min, the loss of glucose proceeds at the same rate with or without sodium‐fluoride. The American Diabetes Association (ADA) guideline on laboratory testing in diabetes recommends that samples be immediately immersed in ice slurry and analysed within 30 min of collection. This is difficult to achieve in routine patient care and is not always followed in GDM testing. The diagnostic criteria for GDM were based on the Hyperglycaemia and Adverse Pregnancy Outcome study, which had strict protocol for glucose sample handling. In a study by Daly et al., implementation of ADA glucose sample handling recommendations resulted in a 2.7‐fold increased detection of GDM compared with usual hospital practices. When studying the impact of handling fluoride‐oxalate samples at room temperature in rural and remote Australia, Jamieson et al. estimated a 62% under‐diagnosis of GDM compared to fluoride‐oxalate samples on ice slurry. Fluoride‐citrate‐EDTA (ethylenediaminetetraacetic acid) tubes virtually inhibit glycolysis and have been recommended to replace sodium‐fluoride‐containing tubes. However, these tubes are not universally available and the costs are significantly higher.

The biological variations of FBG and OGTT should not be overlooked. Chai et al. examined the impact of within‐individual biological variation (CVi) and analytical variation on classification of diabetes in a large non‐pregnant population cohort in Singapore. Using numerical simulations, accounting for CVi for FBG (5.7%) and OGTT (16.7%) and analytical variations, they found a potential 14.4% increase in diabetes diagnosis if glucose measurements were repeated for the group with original classification of impaired fasting glucose (World Health Organization criteria). Furthermore, a potential 26.6% increase in diabetes diagnosis was estimated if glucose measurements were repeated for the group with impaired glucose tolerance. The use of the average of results of the repeat laboratory tests appears to improve the consistency of disease classification.

The COVID‐19 pandemic has led to a revisit of laboratory testing. We should never waste a good crisis.