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

10. Blood chemistry 325Table 10.1 Blood glucose concentrations in fasting patientsFluidGlucose concentrationSI units (mmol/l)Traditional units (mg/100ml)Venous blood 3.3–5.5 60–100Capillary blood 3.9–5.5 70–100Serum 3.9–6.4 70–115Plasma 3.9–6.4 70–115Glucose concentrations higher than 16.5mmol/lDilute solutions B (blank) and P (patient) with an equal volume of glacial aceticacid. Using diluted solution B in the cuvette, set the colorimeter reading to zero.Then read the absorbance A Pwith diluted solution P in the cuvette. Recalculate theglucose concentration, using the new value of A Pand the value of A Rthat wasobtained previously. Multiply the result by two (because solution P has beendiluted 1 in 2) to obtain the true glucose concentration.Glucose concentrations lower than 2.3mmol/lRepeat the entire test. In step 1, use 1.6ml of trichloroacetic acid solution (insteadof 1.8ml), and in step 2 use 0.4ml of blood, serum or plasma (instead of 0.1ml).Perform the test and calculate the result exactly as before. Divide the result by fourto obtain the true glucose concentration.10.2 Estimation of urea concentration in blood: diacetylmonoxime/thiosemicarbazide methodUrea is a waste product formed in the liver following the breakdown of protein. Itpasses into the blood, is filtered out by the kidneys and excreted in the urine.If the kidneys do not remove urea, the concentration in the blood is increased. Thiscan happen if the kidney tubules become damaged or if the volume of blood flowingthrough the kidneys is reduced.10.2.1 PrincipleThe proteins are first precipitated by trichloroacetic acid. The urea in the filtratereacts with diacetyl monoxime in the presence of acid, oxidizing reagent andthiosemicarbazide to give a red solution. The colour is measured using a photoelectriccolorimeter.10.2.2 Materials and reagents●●●●ColorimeterConical tubes and test-tubes (to hold 20ml)Pipettes, 50ml, 0.1ml, 0.5ml, 5mlMeasuring cylinder, 50ml● Water-bath at 100°C● Urea reagents (reagent no. 62):— trichloroacetic acid, 5% solution— diacetyl monoxime stock solution— colour reagent— urea stock reference solution (125mmol/l)— urea working reference solution (10mmol/l)● Acid reagent (reagent no. 6)