tests are then automatically prioritorised into the most efficient order and the analyser
programmed to take the appropriate volume of sample by means of a sampler that may
also be capable of detecting microclots in the sample, add the appropriate volume of
reagents in a specified order and to monitor the progress of the reaction.Internal
quality controlsamples are also analysed on an identical basis at regular intervals.
The analyser automatically monitors the use of all reagents so that it can identify when
each will need replenishing. When the test results are calculated, the operator can
validate them either on the analyser or on the main computer database. When appro-
priate, the results can also be checked against previous results on the same patient.Micro sensor analysers
Recent advances in micro sensor technology have stimulated the development of
miniaturised multi-analyte sensor blocks that are widely used in the clinical biochem-
istry laboratory for the routine measurement of pH, pCO 2 ,Naþ,Kþ,Ca^2 þ,Cl, glucose
and lactate (Section 1.3.5). The basic component of the block is a ‘cartridge’ that
contains the analyte sensors, a reference electrode, a flow system, wash solution, waste
receptacle and a process controller. The block is thermostatically controlled at 37Cand
its surface provides an interface to the analyte sensors (Fig. 16.2a). The sensors are
embedded in three layers of plastic, the size and shape of a credit card. Each card may
contain up to 24 sensors. A metallic contact under each sensor forms the electrical
interface with the cartridge. As the test sample passes over the sensors, a current is
generated by mechanisms specific to the individual analyte and recorded. The size of
the current is proportional to the concentration of analyte in the fluid in the sample
path. Calibration of the sensors with standard solutions of the analyte allows the
concentration of the test sample to be evaluated. The sensor card and the sample path
are automatically washed after each test sample and can be used for the analysis of up
to 750 whole blood samples before being discarded.
The glucose and lactate sensors have a platinum amperometric electrode with a
positive potential relative to the reference electrode. In the case of the measurement of
glucose, the glucose oxidase reacts with the glucose and oxygen to generate hydrogenTable 16.3Examples of analytical techniques used to quantify
analytes by auto-analysersAnalytical technique Examples of analytes
Ion-selective electrodes Kþ,Naþ,Liþ,Cl
Visible and UV spectrophotometry Urea, creatinine, calcium, urate
Turbidimetry IgG, IgA, IgM, D-dimer (a metabolic product
of fibrinogen)
Reaction rate Enzymes – AST, ALT, GGT, AP, CK, LD,
Enzyme multiplier immunoassay
test (EMIT)Therapeutic drug monitoring – phenytoin,
carbamezapine634 Principles of clinical biochemistry