ULTRA
SENSITVE PSA
Detection
limits of the ultra-sensitive PSA assays is a long and complicated subject.
One
of the key issues is what is termed "background noise" . This refers to the fact
that PSA is NOT prostate specific despite it beng named as Prostate Specific Antigen.
Other glands in the body can and do produce this protein and when measuring the
minuscule quantities in ultra-sensitive assays after radical prostatectomy, the
PSA from other organs can occasionally be confused with PSA generated from prostate
cancer or any recurrence of the disease. Again it is important to bear in mind
that PSA is NOT
prostate cancer specific.
A good deal of the questionable value of
ultra-sensitive assays can be traced back to the mid-1990's around the time Diagnostic
Products began marketing their Immulite 3rd generation assay when the competition
between assay manufacturers resulted in misleading sales pitches.
In 1996
Dr. Thomas Stamey of Stanford wrote an interesting essay for the medical journal
"Clinical Chemistry" about the resulting confusions regarding assay sensitivity
titled "Lower limits of detection, biological detection limits, functional sensitivity,
or residual cancer detection limit? Sensitivity reports on prostate- specific
antigen assays mislead clinicians." (Clin Chem. 1996 Jun;42 (6 Pt 1):853-7) Here
are some quotes from the article:
"Unfortunately, commercialism has led
assay manufacturers to tout their differences in LLD (lower limit of detection)
among assays to the point where laboratorians believe that the LLD has clinical
significance and subsequently relay these values to the clinicians on the laboratory
report...'
"Many assays use bovine serum albumin (BSA) as their matrix,
although other animal sera are also used. However, BSA is a highly purified simple
protein that bears no resemblance to the complexities of human serum. Intra-assay
variation with such simple proteins might be elegantly low, but this would be
misleading for the clinician and potentially harmful for the patient..."
In
a research lab setting, greater pains are taken to assure consistent quality of
reagents and to fine tune calibration of the instruments, and they can accurately
measure PSA down to a level of 0.01 when using a better assay such as the Immulite.
One hears of PSA assays that can measure levels down to around .002, but this
kind of accuracy is not possible in a medium as complicated as human blood. This
level of accuracy is possible only when PSA is suspended in a simpler medium such
as bovine serum albumin.
There is also the question of accuracy in measuring
volumes. In an exchange on a Mailing List, one member said: " I was ' under 0.1'
and asked for an ultra-sensitive test to see just how low it was. the 1st test
with the more sensitive scale was 0.009, and is now 0.003. I find it a miracle
that lab workers can even measure down to a trillionth of a gram per ml."
The
response, from a man whose career was in measurement at Princeton, Argonne National
Labs, and Fermilab, said in part: "Nothing on earth can be measured to this precision.
Well, there are some obscure things (besides time) that only physicists care about.
But think about it. To measure the PSA, one must take a volume sample. The above
ml. Now how accurately can you do that? Think about measuring gasoline or milk.
It is hard to do it to 1%. ……ignore any small changes. You just can't measure
them."
The other issue is the calibration. Here, there are issues like
what standard was used, how long ago the instrument was calibrated, and what the
environmental sensitivity of the instrument is. For example, if the instrument
was calibrated at one temperature and is used at another temperature, then all
the measurements can be off by some fixed amount (such errors are called systematic
errors). To prove to yourself that instruments are not perfect, look at the thermometers
being sold in a store. Some will show, e.g., 71 deg F, some 73, some 72, etc.
Assuming
that a laboratory's instrument is calibrated and used properly (a big assumption),
one way to get an idea of the "accuracy" of the reported result is to, say, divide
a large blood sample into 100 equal parts and submit each as if it were a different
sample. The results may come back as 0.002, 0.001, 0.002, 0.003, 0.005, 0.001,
etc. The average of the 100 measurements may be 0.002, but, for any given sample,
the result may be slightly different - not because the sample was different but
because of measurement noise (or random errors). Generally, it is not practical
for commercial labs to calibrate their instruments and cope with differences of
batches of reagents well enough to squeeze maximum possible accuracy out of the
ultra-sensitive assays, so they will draw an arbitrary line, and report nothing
lower than about 0.03. Different labs may choose slightly different numbers, even
though they may be using the same assay.
A laboratory should report the
results as a value plus an uncertainty, e.g., 0.002 plus or minus 0.001 (written
as +/- 0.001). What uncertainty is reported depends on the "confidence interval."
If the uncertainty is the standard deviation of the measurements, then that means
that 2/3 of the measurements will be within that uncertainty.
If it is six
times the standard deviation, then 99% of measurements will be within the uncertainty
(i.e., one percent of the time, the measurement will still be in error by more
than the uncertainty reported.) Does anyone know of a lab that reports both the
results and the uncertainty in the results? Maybe the labs are trying to protect
us from the complexities of measurement science.
So, if a PSA reading goes
from 0.002 to 0.003, the "increase" may NOT be due to any real increase. It could
be due to just measurement noise.
Analytical labs switch assays often.
This can play havoc with patients (an their docs) if they are not totally aware
of these changes and the potential implication of these changes. Ignoring human
errors (of which there are many) the actual design of the assays and their calibration
standards are fundamentally important. The reason for that is related to the antibodies
used in the design. Some assays use monoclonal antibodies and some use polyclonal
ones. These might detect the different forms of immuno-reactive PSA in different
ways based on the composition of the calibration standard.
The important
thing is to try to get all tests done at the same lab, using the same assay -
though there has been some discussion of standardization in the industry and a
protocol - the so-called Stanford Protocol - was agreed for cPSA the highly sensitive
assays are not standardized relative to each other so simply switching assays
could result in a surprising jump even with no true change. An universal calibrator
would surely reduce the inter assay variability while the intra variability depends
more on sample handling and the physiological variation of PSA in patients.
It
has become harder and harder to stick with the original assay because of Labs
and health insurance changes. If a significant change is noticed, it is worthwhile
to do a baseline comparison using the old and new assays on the same blood sample.
This piece was written after an extensive discussion
on the PPML Mailing List and thanks are due to the wise men who participated and
shared their knowledge.
