Gisborne Hospital Report - Health and Disability Commissioner

Gisborne Hospital 1999 – 2000
PSA Testing Procedures
should then carry out to determine and rectify the cause of the out of control
measuring system, before attempting to repeat the tests.
2.11 Internal quality control samples are often provided by the manufacturer of the
instrument and/or reagents being used, although suitable control samples are
also often available from an alternative manufacturer. There may be
advantages in using an alternative source for control samples, as an
independent check on the instrument or reagent manufacturer’s system.
Alternative material is also sometimes cheaper, more stable, or may contain
other analytes which can be used to check other tests carried out in the
laboratory. The use of an alternative source for internal control samples, as
used for PSA at Gisborne Hospital, is an acceptable practice.
2.12 The error made at Gisborne Hospital was that the stated values for PSA for the
internal control samples were changed to fit the results obtained when the
wrong calibrator was used. The wrong internal control result should have
alerted them to an analytical system error and led to a series of procedures that
would eventually have located the cause of the abnormal control results.
2.13 Internal control samples are analysed frequently to ensure that the analytical
process is under control at all times. Because the internal control values were
altered at Gisborne Hospital, the error introduced into patient specimen results,
as a result of incorrect calibrators being used, remained undetected until a
locum technologist recognised the error.
2.14 There is reference later in this chapter to the use of rules for determining
whether the values obtained on internal quality control samples indicate that
the patient results should be accepted or rejected. There are a number of
statistical techniques, of varying complexity, for doing this. The Westguard
rules and Levey-Jennings plots used at Gisborne Hospital are commonly used.
These techniques were applied by entering the internal quality control sample
results into a software programme (QC Reporter 2.0).
2.15 The reason that rules are used to decide on rejection or acceptance of results,
rather than immediate rejection if one of the control results is not exactly right,
is that there is an unavoidable variation in the precision of all steps employed
as part of the testing process. This in turn produces an unavoidable variation
in the final control results. The rules are used to decide when this variation is
excessive and clinically unacceptable, and thus alerts the technologist to
problems with the instrument or method.
2.16 It is essential that the internal quality control results are checked before the
patient results are reported, as an unacceptable performance should cause
those results to be discarded. Although the rules for acceptance or rejection of
results used at Gisborne Hospital were applied correctly, the internal control
results were not entered into the QC Reporter 2.0 programme until after the
patient results had been reported, and thus failed to fulfil their primary
purpose.
External quality assurance programmes
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