1. Overview
Glucose is the primary source of energy for the body’s cells and is essential for normal physiological functioning. It is obtained directly from the food we consume or synthesized by the body from other substrates. Once absorbed into the bloodstream, it enters body cells with the help of insulin, a hormone secreted by the pancreas.
In the absence or deficiency of insulin, it cannot effectively enter cells and therefore accumulates in the bloodstream, leading to hyperglycemia. Blood glucose levels must be maintained within a narrow range, a balance regulated mainly by the liver in response to the pancreatic hormones insulin and glucagon. This balance is crucial for energy metabolism, particularly for the brain, which relies almost exclusively on glucose for energy.
2. Functions of Blood Glucose
Blood glucose serves several vital physiological functions. It is an important source of energy and provides nutrients to the body’s organs, muscles, and nervous system. it is the primary energy source for the brain and is essential for cellular respiration.
Excess glucose can be stored in the body in various forms. It may be stored as starch, converted into fat and oil, or used in the synthesis of amino acids and structural components such as cellulose. These metabolic pathways ensure energy availability during fasting or increased demand.
3. Indications for Estimation
Blood glucose estimation is one of the most commonly performed laboratory tests. It is indicated in:
- All preoperative cases
- All antenatal cases
- Routine health check-ups
- Patients with symptoms suggestive of diabetes
- Monitoring of known diabetic patients under treatment
- Assessing response to diet, drugs, and insulin therapy
- Suspected insulinoma or glucagonoma
- Conditions such as sickness, stress, and steroid therapy
- Adjustment and management of insulin dosage
4. Sample Collection and Reference Ranges
Accurate glucose estimation requires proper sample handling. Blood glucose decreases rapidly after collection due to ongoing glycolysis by blood cells. Therefore, samples must be tested promptly. To prevent glycolysis, blood for glucose estimation is collected in sodium fluoride bulbs (grey-cap tubes).
Reference ranges (mg/dL):
- Fasting (minimum 8 hours):
- Normal: 70–100
- Prediabetic: 100–125
- Diabetic: >125
- Postprandial (2 hours after meal):
- Normal: 100–140
- Prediabetic: 141–199
- Diabetic: >199
- Random:
- Normal: 70–100
- Prediabetic: 100–125
- Diabetic: >125
5. Methods of Estimation
Blood glucose can be estimated using:
- Spectrophotometry
- Electronic glucose meters
- Continuous glucose monitoring (CGM) systems, which use a small sensor inserted under the skin to measure these levels at frequent intervals
6. Causes and Risk Factors
Hypoglycemia (Low Blood Glucose) may occur due to excessive insulin use, inadequate carbohydrate intake, improper timing of insulin, increased physical activity, alcohol consumption, prolonged starvation, hormonal changes, nesidioblastoma, or insulin resistance syndrome.
Symptoms include fast heartbeat, sweating, shaking, dizziness, hunger, anxiety, irritability, and confusion. Asymptomatic hypoglycemia is particularly dangerous and more common in long-standing diabetics or patients on beta-blockers.
Night-time hypoglycemia may result from high physical activity during the day, excessive insulin, or alcohol intake at night. Preventive measures include regular meals and bedtime snacks when needed.
Neonatal hypoglycemia is defined as plasma glucose <30 mg/dL in the first 24 hours and <45 mg/dL thereafter. Risk factors include prematurity, perinatal asphyxia, and low birth weight.
Hyperglycemia (High Blood Glucose) may occur due to illness, stress, overeating, insufficient insulin, poor compliance with oral drugs, long-term steroid use, or glucagonoma. Common symptoms include fatigue.
7. Limitations
Blood glucose measurement may be affected by application errors, extreme environmental conditions, abnormal hematocrit values, and medication interference. Such factors can lead to inaccurate readings and potentially incorrect treatment decisions, including improper insulin dosing.
