The Oral Glucose Tolerance Test: The Backbone of Gestational Diabetes Detection

Gestational diabetes mellitus (GDM) poses significant health risks for both mother and baby during pregnancy. As a result, using an effective screening method is crucial for early detection and management. The Oral Glucose Tolerance Test (OGTT) stands out as the gold standard test for GDM. The OGTT is the only test recommended by the World Health Organization (WHO), the National Institute for Health and Care Excellence (NICE), the American Diabetes Association (ADA) and the International Federation of Obstetrics and Gynaecology (FIGO). In this blog post, we delve into why the OGTT holds such importance in identifying GDM and ensuring optimal maternal and foetal health outcomes and why it is the only recommended diagnostic test for GDM. 

What is gestational diabetes?

GDM is characterised by elevated blood sugar levels first recognised during pregnancy. Without proper management, it can lead to complications such as macrosomia (excessive foetal growth), birth injuries, pre-eclampsia, and an increased risk of caesarean section. Moreover, mothers with GDM, and their children, have a substantially higher risk of developing type 2 diabetes. Given these risks, timely and effective screening is essential. 

While various diabetes tests are available, including fasting plasma glucose (FPG), random plasma glucose (RPG) and HbA1C, the OGTT is the only recommended diagnostic test for GDM according to standards of care globally. 

Why is the OGTT the gold standard for GDM diagnostic? 

• Sensitivity: 
  The OGTT offers superior sensitivity compared to other screening tests. By measuring blood glucose levels before and after the consumption of a standardised glucose drink, OGTT provides a more comprehensive assessment of glucose metabolism, reducing the likelihood of false-negative test results. 
• Specificity: 
  The OGTT is more specific compared to other screening tests, reducing the likelihood of false-positive test results. Because other diabetes tests are less sensitive, the tests’ “cut-off” values for diagnostic glucose levels have to be adjusted in order to reduce missed cases of GDM by increasing the sensitivity. As the cut-offs are lowered to classify more people with GDM, more people without GDM are caught in the GDM net and the test becomes less specific. The OGTT works differently, allowing high sensitivity and high specificity at the same time.  

Challenges with the in-clinic OGTT

Despite its necessity, the traditional in-clinic OGTT is not without limitations. It is time and labour intensive for healthcare professionals (HCP) and this morning-only test requires fasting, making it difficult for patients to access. Moreover, there is the major issue with sample degradation due to incorrect sample processing and testing, which can lead to significant false negative diagnoses - this will be explored in a separate article. Every one of these disadvantages, however, is eliminated with OGTT home testing, as the blood samples are analysed immediately, the mothers-to-be can do the test from the comfort of their own home, and as a managed service it requires minimal involvement of HCPs time. 

In conclusion, the OGTT remains the cornerstone of gestational diabetes screening, endorsed by leading health organisations and clinical bodies. Its high sensitivity and specificity make it indispensable for early identification and management of gestational diabetes, ultimately improving outcomes for both mothers and babies. While challenges exist with the in-clinic OGTT, the benefits, particularly with the advent of OGTT home testing, far outweigh its limitations, emphasising its central role in antenatal care.