GO
BACK TO: DIAGNOSIS: PSA:
ULTRA
SENSITIVE 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 being 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 - see A
SHORT ESSAY ON PSA.
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 - or check out the precise
time on a digital watch with some of your friends. Most watches show the time
down to one tenth or even one hundredth of a second - and all claim accuracy.
But it is extremely unlikely that five or six watches will match time that precisely.
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 substantialy 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 range with a midpoint - for example between 0.001 and 0.003 with
a mid-point of 0.002 or as a value plus an uncertainty, e.g., 0.002 plus or minus
0.001 (written as +/- 0.001). What range and uncertainty is reported depends on
the "confidence interval." If the uncertainty is the standard deviation of the
measurements, then that means that 65% of the measurements will be within that
uncertainty (and of course 35% will be outside). 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.) although the range will be greater. I am not aware of any laboratory
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 (and their doctors) 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 Although there has been some discussion of standardization
in the industry and a protocol - the so-called Stanford Protocol - was agreed
for total PSA, the highly sensitive assays are not standardized relative to each
other. This means that 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.
Added
November 2011: This study carries an important caveat for the use of ultrasensitive
PSA tests - hopefully men will become aware of this before making any precipitate
decisions based on what may not be as accurate a test as they believed:
J
Urol. 2011 Oct 17. [Epub ahead of print] Poor Agreement of Prostate Specific Antigen
Doubling Times Calculated Using Ultrasensitive Versus Standard Prostate Specific
Antigen Values: Important Impact on Risk Assessment. Reese AC, Fradet V, Whitson
JM, Davis CB, Carroll PR. SourceDepartment of Urology, UCSF Helen Diller Family
Comprehensive Cancer Center, University of California, San Francisco, San Francisco,
California.
Abstract
PURPOSE:
In men with biochemical recurrence after radical prostatectomy, a rapid prostate
specific antigen doubling time is associated with adverse outcomes, and is often
used to guide the type and timing of salvage therapy. It is unknown whether prostate
specific antigen doubling time calculated in the ultrasensitive range (prostate
specific antigen less than 0.2 ng/ml) accurately reflects measures performed in
the traditional range (prostate specific antigen greater than 0.2 ng/ml).
MATERIALS
AND METHODS: We studied 394 men in a national disease registry of men with
prostate cancer (CaPSURE™) who underwent radical prostatectomy, experienced biochemical
failure, and had prostate specific antigen doubling time assessed using ultrasensitive
and traditional prostate specific antigen values. Agreement between these measurements
was assessed using Cohen's kappa score.
RESULTS:
Median ultrasensitive prostate specific antigen doubling time was 11.9 months
(IQR 6-29) and median traditional prostate specific antigen doubling time was
240 months (IQR 18-240). Agreement between ultrasensitive and traditional prostate
specific antigen doubling time was poor, with a weighted Cohen's kappa score of
0.04 (95% CI -0.02-0.10). Using a dichotomous prostate specific antigen doubling
time cutoff of 9 months, there was a statistically significant difference between
ultrasensitive and standard prostate specific antigen doubling time (exact McNemar
p <0.01). Ultrasensitive prostate specific antigen doubling time was more or less
rapid than traditional prostate specific antigen doubling time by more than 15
months in 244 (62%) and 35 (9%) patients, respectively.
CONCLUSIONS:
Agreement between prostate specific antigen doubling time calculated using ultrasensitive
vs traditional prostate specific antigen values is poor. Ultrasensitive prostate
specific antigen doubling time is often significantly more rapid than traditional
prostate specific antigen doubling time, potentially overestimating the risk of
clinical recurrence. Until the significance of ultrasensitive prostate specific
antigen doubling time is better characterized, the decision to proceed with salvage
therapy should not be based on prostate specific
antigen doubling time calculated using ultrasensitive prostate specific antigen
values.
PMID:22014796 .
The main piece was written after an extensive
discussion on the PPML Mailing List
in 2009 and thanks are due to the wise men who participated and shared their knowledge.
