By Prof Ram Shankar Upadhayaya
When a person’s blood sugar levels are slightly higher than normal, doctors might suspect they have diabetes, but it would be unclear. We often use the term “gray areas” in these situations because the diagnosis is not confirmatory.
This is where the glucose tolerance test becomes incredibly important. It can complement the usual tests like fasting, post-meal blood sugar, and HbA1C, providing additional information when needed.
According to usual standards for diagnosis, a fasting blood sugar level below 100 milligrams per deciliter (mg/dL) is considered normal. If a patient’s blood sugar consistently measures 126 mg/dL or higher in multiple tests, it’s a clear sign of diabetes. However, there are situations where the results may not strongly indicate diabetes, which can cause some uncertainty. It’s worth noting that scientific research has shown that blood sugar levels tend to naturally rise during the early morning hours due to our body’s internal clock, known as the circadian rhythm.
The glucose tolerance test is vital in offering a clearer picture in these situations. This test becomes significant when patients show apparent signs of diabetes but don’t match the traditional diagnostic criteria. By incorporating the glucose tolerance test alongside other evaluations, healthcare providers can make well-founded decisions to confirm or rule out diabetes, ensuring that diagnoses are prompt and precise.
It’s important to note that postprandial (PP) blood sugar levels below 140 mg/dL are within the normal range. Readings between 140 and 199 mg/dL fall into a pre-diabetic category, indicating an increased risk of developing diabetes.
Two hours after eating a meal, a patient’s postprandial (PP) blood glucose level slightly surpasses 200 milligrams per deciliter (mg/dL), or it hits precisely 200 mg/dL. This specific threshold serves as a definite sign of diabetes. It’s important to highlight that a prediabetes diagnosis brings its own set of concerns. Individuals with prediabetes face not only an increased risk of developing type 2 diabetes but also a heightened susceptibility to heart disease, even if diabetes doesn’t eventually emerge.
Coming to the glucose tolerance test, here’s how it typically works: The patient starts by consuming 150-200 grams of glucose over three days to prepare for the test. On the fourth day, they come in for the test after fasting since midnight the previous night. The test begins with the patient swiftly drinking a solution containing 75 grams of glucose dissolved in 300 milliliters of water. Two critical readings are taken at the start (time zero) and another at the 120-minute mark, post-meal.
The key to diagnosis lies in interpreting the results. If the glucose levels at zero and 120 minutes after consumption measure 140 mg/dL or lower, it falls within normal range. However, if the readings are between 140 and 199 mg/dL, it indicates prediabetes or an increased risk of diabetes. If the reading equals or exceeds 200 mg/dL, it confirms diabetes. Hence, this method can tandemly validate the standard fasting or PP tests.
HbA1C is another diagnostic tool to determine diabetes. To grasp the mechanics behind this assessment, consider hemoglobin A1C, a protein comprising alpha and beta globin chains. Glucose adheres to beta chains, initiating a process known as glycation via a non-enzymatic process that occurs spontaneously in blood. The letter ‘C’ in A1C signifies “glycated hemoglobin”, wherein “A1C” represents carbohydrates, specifically glucose. This glucose-bound hemoglobin predominantly consists of hemoglobin A1. Hence, the amount of HbA1C is directly proportional to the amount of glucose in the blood.
It’s crucial to recognize that hemoglobin resides within red blood cells (RBCs), serving as a gauge for glucose levels throughout an RBC’s typical lifespan, usually around 120 days. Nevertheless, this duration can vary depending on individual circumstances.
An important concept to understand here is that A1C levels provide a more precise reflection of the past month rather than an all-encompassing view of the preceding three months.
In essence, hemoglobin A1C most accurately represents the glucose levels of the last month. However, it generally contributes to the overall assessment of a three-month or 120-day period.
Now, let’s explore the numerical thresholds. In healthy individuals, glycated hemoglobin, or A1C, usually stays between 4% and 5.6%. This is a measure of how much glucose has attached to your hemoglobin. Within the 5.7% to 6.4% range, glycated hemoglobin corresponds to prediabetes or an elevated risk level. Finally, if your A1C test shows 6.4% or higher, it’s a clear sign of diabetes. This means we need to discuss how to manage your health.
Certain patients may encounter anomalies in the lifespan of their RBCs, potentially leading to premature RBC degradation or other hemoglobinopathies. In such cases, the three-month assessment may not be applicable. Instead, aligning the evaluation with the specific lifespan of the patient’s RBCs becomes essential. Vitamin and iron deficiencies and certain drugs like aspirin may also result in abnormal HbA1C values. Hence, a glucose tolerance test in these cases may prove handy.
Vigilant glucose monitoring is critical in ascertaining a diabetes diagnosis. This entails periodic assessments of ketone levels over several days or weeks to gauge the effectiveness of diabetes management. In cases where patients rely on insulin, it’s imperative to maintain an appropriate insulin dosage to avoid dangerous fluctuations in ketone levels.
As an aid for better diabetes management, today, we have handy tools like continuous glucose monitors and smart insulin pumps with built-in sensors.
Furthermore, the development of an artificial pancreas capable of assessing glucose levels and administering insulin is poised to become more widespread, less invasive, and more user-friendly. This represents a promising direction in the management of diabetes, and that future is rapidly approaching.
(The writer is a US-based medical scientist)
