Gisborne Hospital Report - Health and Disability Commissioner

Gisborne Hospital 1999 – 2000
PSA Testing Procedures
2.17 External quality assurance refers to programmes provided by an independent
external source, such as the Australasian Quality Assurance Programme
(AQAP) provided by the Royal College of Pathologists of Australasia. AQAP
is used by the majority of laboratories in New Zealand as their external QA
programme.
2.18 The programme organisers send out samples at regular intervals (monthly for
PSA) to a large number of subscribing laboratories. The samples have target
values for many different analytes, but these target values are unknown to the
testing laboratory. After analysing the external control sample, the laboratory
submits their results to the organisers. A report is then returned to the
laboratory, which compares their submitted results with those submitted from
all other laboratories, and they are also compared with the target values.
2.19 The external quality assurance programme thus acts as a final safety net for all
other quality control procedures. If a result does not compare closely with the
results obtained by the majority of other laboratories using the same
instrument and same reagents, then it is clear that there is a quality problem.
The presentation of data in the external quality control programme reports
illustrate the type of problem a laboratory may be having.
Calibrators
2.20 A calibrator is a sample containing an accurately measured and known amount
of a particular analyte. When an instrument is calibrated, at least two
calibrators are measured, one with a low value and one with a high value. The
instrument uses the readings it gets from these calibrators to draw a line (or
curve if more than two calibrators), and this curve is then used to relate the
read-out of the instrument to a concentration of the analyte. In effect,
instrument readings on subsequent patient and internal control specimens are
compared with the calibration curve to calculate the concentration of the
analyte.
2.21 If the wrong calibrators are used, as they were at Gisborne Hospital for PSA,
the calibration curve is incorrect and the concentrations of PSA calculated in
internal control and patient specimens are also incorrect. The error with the
internal control results was recognised, but by altering the control values, the
problem was dealt with inappropriately - in effect, ignored - and the errors
continued with patient specimens.
2.22 Because many factors are involved in maintaining quality, a system of records
and documentation is needed for periodic review and evaluation by the
laboratory, but also by agencies such as IANZ. These quality control records
must be maintained for a period of time (often a minimum of two years) to
document that testing has occurred. These records should include:
• Routine maintenance
• Reagent lot numbers in use and expiration dates
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