POCT (point-of-care test) urine test for glucose is mainly based on a specific chemical reaction principle. The most common one is the glucose oxidase method. The following is a detailed introduction to its principle and related situations:
Basic principle
POCT urine test for glucose mostly uses the glucose oxidase-peroxidase method. The membrane block of the test paper contains components such as glucose oxidase, peroxidase and chromogen. The specific reaction process is as follows:
- Step 1: Glucose oxidation reaction: When the urine sample comes into contact with the test paper, the glucose oxidase on the test paper will specifically catalyze the oxidation reaction of glucose in the urine, oxidizing glucose into glucuronic acid and hydrogen peroxide (H₂O₂). This step of the reaction is highly specific, that is, glucose oxidase will only act on glucose, and will not react with other sugars or substances, thereby ensuring the accuracy of the test. The chemical reaction formula is: C₆H₁₂O₆ (glucose) + O₂ (oxygen) $\stackrel{glucose oxidase}{→}$ C₆H₁₀O₆ (glucuronic acid) + H₂O₂ (hydrogen peroxide).
- Step 2: Oxidation reaction of hydrogen peroxide: The generated hydrogen peroxide releases nascent oxygen [O] under the catalysis of peroxidase. Nascent oxygen has strong oxidizing properties and can cause the color of the chromogen in the test paper to change. Different chromogens will show different colors after being oxidized, and the depth of the color is proportional to the content of glucose in the urine. For example, common chromogens may change from colorless to blue, green or brown after the reaction. By comparing with the standard colorimetric card, the content of glucose in the urine can be roughly determined. Its chemical reaction formula is: H₂O₂ $\stackrel{peroxidase}{→}$ H₂O (water) + 【O】 (primitive oxygen); 【O】+ chromogen $\stackrel{}{→}$ oxidized chromogen (colored substance).
Clinical significance
- Hyperglycemic glycosuria: When the blood sugar level of the human body rises and exceeds the glucose reabsorption threshold of the kidney (generally 10mmol/L), the renal tubules' ability to reabsorb glucose reaches saturation, and the excess glucose will be excreted with urine, resulting in positive urine glucose. It is common in diabetic patients. Due to insufficient insulin secretion or insulin resistance, blood sugar cannot be effectively utilized and stored, resulting in increased blood sugar. In addition, high-sugar diet, craniocerebral trauma, emotional excitement and other situations may also cause stress-induced blood sugar increase and positive urine glucose.
- Normal blood sugar glycosuria: In some cases, even if the blood sugar level is normal, urine glucose may be positive. This is mainly due to the reduced renal tubular reabsorption of glucose, resulting in a lower renal glucose threshold. For example, some kidney diseases can affect the function of the renal tubules, reducing their ability to reabsorb glucose; in late pregnancy and lactating women, due to physiological changes, the renal glucose threshold may also be lowered, resulting in positive urine glucose.
Precautions
- Influence of interfering substances: Since the detection process involves redox reactions, some substances in the urine may interfere with the test results. For example, when the urine contains substances with stronger reduction ability than pigments, such as vitamin C, the test results may be low or even false negatives; high concentrations of ketone bodies in the urine can also cause false negatives.
- Other influencing factors: Increased urine specific gravity can reduce the sensitivity of the test strip to urine glucose; false positives may occur when urine is contaminated by strong oxidizing substances such as peroxides or hypochlorites. In addition, the test uses an enzymatic reaction, so the test results are related to temperature and time. The colorimetric test should be performed at the specified temperature and time, and the test strip should be properly stored in a cool and dry place to ensure the accuracy of the test results.