1. Take six tubes and label them ‘B’ (reagent blank) and 1 to 5. Pipette 1.5 ml of protein precipitant reagent into each tube. Note: The use of the protein precipitant reagent is required because it forms part of the buffer system and contains phenol which is needed for the colour reaction.
Add to each tube as follows:
Tube
B …………………….. 0.05 ml distilled water
1 ……………………. 0.05ml standard, 2.5 mmol/l
2 ……………………..0.05ml standard, 5 mmol/l
3 ……………………..0.05ml standard, 10 mmol/l
4 ………………………0.05ml standard, 20 mmol/l
5 ……………………..0.05ml standard, 25 mmol/l
3. Continue as described in steps 5 to 7 of the glucose method.
4. Take a sheet of graph paper and plot the absorbance of each standard (vertical axis) against its concentration in mmol/l (horizontal axis). A linear calibration should be obtained.
The useful working limit (linearity) of the glucose oxidase method is about 25 mmol/l.
Check the calibration graph by measuring a control serum. A new calibration graph should be prepared and checked against controls whenever stock reagents are renewed.
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