Is Proficiency Testing Enough to Prove Accuracy?
Proficiency Testing (PT) is just part of life
for most laboratories. Laboratories are sent challenges (unknown samples) which
they test like patient samples and then submit their results. An agency like CAP then grades the result against
those submitted by peers using the same test system. Your lab is considered as doing a good job if your results are close to those of the peer
like standard deviation and SDI give the impression the results generated are
being checked for degree of accuracy. But are they? Well not really, the mean
is used as a surrogate for the correct value and the variation from that mean
is then regarded as the degree of inaccuracy (bias).
PT materials are not patient samples and often additional material is added to preserve
and stabilize the samples. How do these additives change how samples react? To assume
that closeness-to-peer indicates “correct” accurate results is to be overly
and even shipping and storage conditions- introduce matrix and other effects meaning the PT sample
used to give you that “atta boy” is in fact not directly comparable or
commutable to a fresh sample from your patient.
what if we just made allowance for that knowledge, knowing PT is not perfect
but it’s the best we can do right now? However that assumption raises another issue. How can
we be completely sure that our test system (say an analyzer with a certain
reagent) is in fact producing accurate patient results. Calibrators and quality control
material often have matrix effects as well. Then there is the additional issue of
comparability among various test systems.
research shows that a clinical condition has certain cut points or diagnostic targets.
Given the degree of uncertaintity and variation how can a clinician be
confident making clinical decisions based on such results? We are already
familiar with harmonization for cholesterol and with the use of INR (versus
PT) to guide warfarin anticoagulation.
In both cases there is much less inter-laboratory variation than there used to be.
brings us to the areas of accuracy and harmonization. Accuracy is “trueness of
result” while harmonization is a system of ensuring results for an analyte
are truly comparable; even performed by different methods, at different sites, on different platforms.
has recently introduced Accuracy-Based Surveys for various analytes like
lipids, Vitamin D, Creatinine, testosterone and Hemoglobin A1C.
is a totally voluntary process and laboratories might not want to subject
themselves to extra scrutiny and the possibility of “failing” even if results are
within the peer group. One advantage of the Accuracy Based Program (ABP) is
that manufacturers can re-evaluate their products (reagents and test system)
and even revisit their calibration traceability.
it be wonderful if your PT results indicated how you were doing in an absolute
sense, and not simply compared to your peer group who may have the same bias? Wouldn’t
it be great to have tests systems giving equivalent results across platforms
and laboratorians and clinicians having the confidence that their “numbers” are both
accurate and comparable and can be used to confidently guide patient care