What are the glucose concentrations of samples with absorbances 0.681 and 1.120 using a 400 mg/dL standard with absorbance 1.425?

Calculating Glucose Concentrations Using Absorbance Values

To calculate the glucose concentrations of the samples, we need to use direct proportion since absorbance is directly proportional to concentration in spectrophotometric assays.

Solution Method

When using spectrophotometric methods for glucose measurement, the Beer-Lambert law applies, which states that absorbance is directly proportional to concentration 1. This relationship allows us to calculate unknown concentrations using a simple proportion:

Concentration of unknown = (Absorbance of unknown / Absorbance of standard) × Concentration of standard

Calculations:

For the standard:

• Glucose concentration: 400 mg/dL
• Absorbance: 1.425

For Sample A:

• Absorbance: 0.681
• Glucose concentration = (0.681 / 1.425) × 400 mg/dL = 191.2 mg/dL

For Sample B:

• Absorbance: 1.120
• Glucose concentration = (1.120 / 1.425) × 400 mg/dL = 313.7 mg/dL

Importance of Accurate Glucose Measurement

Accurate glucose measurement is critical for proper diagnosis and management of diabetes mellitus. According to guidelines, laboratory methods for glucose measurement are well standardized with imprecision (CV) ≤2.6%, using primarily hexokinase or glucose oxidase methods 1.

The American Diabetes Association recommends that glucose measurement should have:

• Analytical imprecision ≤2.4%
• Bias ≤2.1%
• Total error ≤6.1%

Key Considerations for Glucose Assays

When performing glucose assays, several factors can affect accuracy:

1. Sample handling: Proper sample handling is essential to prevent glycolysis. Samples should be placed immediately in an ice-water slurry and plasma separated within 30 minutes 1.

2. Method standardization: Enzymatic methods (hexokinase or glucose oxidase) are most commonly used and well standardized 1.

3. Calibration: Proper calibration using appropriate standards is crucial for accurate results.

4. Matrix effects: The sample matrix (whole blood, plasma, or serum) can affect glucose measurements. Glucose concentrations in plasma are approximately 11% higher than in whole blood with normal hematocrit 1.

Clinical Implications

The calculated glucose values have clinical significance:

• Sample A (191.2 mg/dL): This value is elevated above the normal fasting range (70-110 mg/dL) and falls within the range consistent with diabetes (≥126 mg/dL fasting or ≥200 mg/dL random) 2.
• Sample B (313.7 mg/dL): This value is significantly elevated and consistent with poor glycemic control in diabetes 2.

Accurate glucose measurement is essential for proper diagnosis, treatment decisions, and monitoring of diabetes management, directly impacting patient morbidity, mortality, and quality of life.