Page 365 - The Veterinary Laboratory and Field Manual 3rd Edition

334 Susan C. Cork, Willy Schauwers and Roy Halliwell

Figure 7.5 Light reflected, absorbed and transmitted when it falls on a coloured solution.

However, this is only true if the relationship Practical aspects
between absorbance and concentration is linear
(see next section, Figure 7.6). The reliability of colorimetry based tests depends
For biochemical tests that come in kit form, on the uniformity of the solution to be tested
the wavelength required to read the test will be and the suitability of the equipment used. The
recommended in the test protocol provided. A cuvettes used must be of good quality and free
standard reference reagent will also be provided of smears or dust. Plastic cuvettes should not be
to allow calibration. Even with kit tests it is re-used. Some glass cuvettes may be re-used if
necessary to standardize each test by running a properly cleaned.
‘blank’ sample which contains all the reagents In most cases it is necessary to prepare a stan-
for the test except the test substance. dard curve to calibrate equipment for specific
A standard (that is, reference) solution of colorimetric tests. The procedure is outlined
known concentration, along with the ‘blank’ below.
allow appropriate calibration of the colorimetry
equipment. This should be done before each • Prepare serial dilutions of a known amount
batch of samples is processed and is an essential of the substance to be measured and add the
part of the quality control process. test kit reagents.

Figure 7.6 Calibration graph showing
the potential linear or exponential
relationship between absorbance
readings and concentration.

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