peroxide which then diffuses through a controlling layer (Fig. 16.2b) and is oxidised
by the platinum electrode to release electrons and create a current flow, the size of
which is proportional to the rate of hydrogen peroxide diffusion.
The GEM Premier 4000 system contains an active quality control process controller
that monitors the operation of the system, validates the integrity of the cartridge and
monitors the electrode response to detect microclots in the test sample that may
invalidate the analytical results.Immunoassay section
Immunoassay procedures undertaken by modern auto-analysers are mostly based on
fluorescence or polarised fluorescence techniques. The range of analytes varies from
manufacturer to manufacturer but usually involves basic endocrinology (e.g. thyroid
function tests), therapeutic drugs (theophylline, digoxin) and drugs of abuse (opiates,
cannabis). The operation of auto-analysers in immunoassay mode is similar to that
described above and the results are generally reported on the same day, and are
generally compared with the previous set of results for the patient.Manual assays section
This approach to biochemical tests is generally more labour intensive than the other two
sections and covers a range of analytical techniques such as acetate or gel electrophor-
esis, immunoelectrophoresis and some more difficult basic spectrophotometric assays.
Examples include the assays for catecholamines (for the diagnosis of phaeochromocy-
toma), 5-hydroxyindole acetic acid (for the diagnosis of carcinoid syndrome) or HbA1c
(for the monitoring of diabetes).Result reporting
The instrument operator or the section leader initially validates analytical results. This
validation process will, in part, be based on the use of internal quality control proced-
ures for individual analytes. Quality control samples are analysed at least twice daily or
are included in each batch of test analytes. The analytical results are then subject to an
automatic process which identifies results that are either significantly abnormal or
require clinical comment or interpretation against rules set by senior laboratory staff.Neonatal screening
Neonatal or newborn screening is the process of testing newborn babies for certain
potentially dangerous disorders. If these conditions are detected early, preventative
measures can be adopted that help to protect the child from the disorders. However,
such testing is not easy due to the difficulty of obtaining adequate samples of biological
fluids for the tests. The development of tandem MS techniques has significantly
alleviated this problem. It is possible to screen for a range of metabolic diseases using
a single dried bloodspot 3 mm in diameter. There is no need for pre HPLC separation of
the sample but the technique is used simply to deliver the sample to the MS. A large
number of inherited metabolic diseases can be screened by the technique including
aminoacidopathies such as phenylketonuria (PKU) caused by a deficiency of the
enzyme phenylalanine hydroxylase,fatty acid oxidation defectssuch as medium chain636 Principles of clinical biochemistry