Manual of basic techniques for a health laboratory - libdoc.who.int

324 Manual of basic techniques for a health laboratoryFig. 10.5 Heating the testtubesin a waterbath10. Mix the contents of each tube. Place all the tubes in the water-bath at 100°Cfor exactly 12 minutes (Fig. 10.5).11. Remove the tubes and allow them to cool in a beaker of cold water for5 minutes.12. Measure the colour produced in a colorimeter at a wavelength of 630nm.(a) Place the orange-red filter in the colorimeter.(b) Fill the colorimeter tube or cuvette with the solution contained in the tubemarked B (blank) and place in the colorimeter.(c) Adjust the reading of the colorimeter to zero with the cuvette containingsolution B in place.(d) Pour solution B out of the cuvette, rinse the cuvette with a small amount ofsolution R (reference), pour this out, and fill the cuvette with solution R;place the cuvette in the colorimeter and read the absorbance, A R.(e) Pour solution R out of the cuvette, rinse the cuvette with a small amount ofsolution P (patient), pour this out, and fill the cuvette with solution P;place the cuvette in the colorimeter and read the absorbance, A P.Calibration of the colorimeterBefore taking measurements, prepare a calibration graph using the different concentrationsof the glucose working reference solution treated as described in steps6–9. The graph should be linear up to the highest concentration and should passthrough the origin. Prepare a new graph whenever the o-toluidine reagent is renewed,to confirm the linearity.10.1.4 ResultsCalculationCalculate the concentration of glucose in the blood specimen using the followingformula: 1concentration of glucose in blood mmol l( ) = ( ) ¥A A Cwhere:A P= absorbance reading of the patient’s specimenA R= absorbance reading of the glucose working reference solutionC = concentration of the glucose working reference solution.Note: If a control serum has been included, make the calculation for that serum inexactly the same way, substituting A c(absorbance of the control solution) for A pinthe formula.PRReference rangeThe reference ranges of glucose concentrations in the blood of fasting patients areshown in Table 10.1.High and low valuesIf unusually high or low values are observed, the test should be repeated in order toconfirm the results, as described below.1The calculation given is for SI units. The formula for calculating blood glucose concentrationsin traditional units is as follows:concentration of glucose ( mg 100ml)= concentration of glucose ( mmol l)¥1 0.0555