Partial drawback: iffy APTTs lead to tube’s exit
What laboratories should do
When the hospital laboratories of Hahnemann University and
the Medical College of Pennsylvania in Philadelphia were getting
ready to merge in 1997, Jamie Siegel, MD, then director of both
coagulation laboratories, had to synchronize her procedures. One
change involved switching from standard full-draw (4.5 mL) to partial-draw
(2.7 mL) sodium citrate tubes.
But Dr. Siegel worried that the move to the partial-draw tubes
might affect the accuracy of activated partial thromboplastin times
when monitoring patients on unfractionated heparin therapy. The
preanalytical variables that influence APTT were already known-how
much time passes before a blood sample gets to the laboratory, how
long the laboratory takes to process the sample, which reagent is
used in testing. So when her colleagues Kathleen Sazama, MD, JD,
Vanlila Swami, MD, and David Bernard, MD, PhD, agreed the partial-draw
tube might be another preanalytical variable, Dr. Siegel set out
to compare APTTs from heparinized patients using both types of tubes.
Dr. Siegel evaluated a total of 74 blood samples drawn in the two
institutions and found "huge differences" in some patients with
prolonged APTTs, she told CAP TODAY. While there was no apparent
difference in the results of coagulation testing among normal individuals,
the mean abnormal range in APTT for one hospital was 64.6 seconds
on samples obtained with the full-draw tube and 53.6 seconds with
the partial-draw tube. At the other institution, the mean abnormal
range was 62.7 seconds for the full-draw and 50.1 seconds for the
partial-draw tubes (Am J Clin Pathol. 1998;110:184-187). "If you
look at individual numbers, some samples had up to 30-plus seconds
of a difference, which can be significant when you’re dealing with
heparin," she says.
Getting an APTT that’s 20 or 30 seconds shorter than the gold standard
value obtained with full-draw tubes translates into higher heparin
doses than necessary, says Dr. Siegel, who now is associate professor
of medicine in the division of hematology at Robert Wood Johnson
University Hospital, New Brunswick, NJ. "We didn’t know if that,
in fact, would increase bleeding risk for patients, but we knew
it theoretically could," she points out. And because of that possibility,
the Medical College of Pennsylvania did not make the switch.
Partial-draw sodium citrate tubes have been around for many years.
The tubes were developed to reduce the amount of blood drawn from
patients who must undergo laboratory testing regularly. The partial-draw
tubes have the same configuration as their full-draw counterparts-13
¥ 75 mm; they collect less blood because they have less interior
Until recently, no one suspected partial-draw tubes introduced
preanalytical variability. Mark Hosler, MD, medical director of
the laboratory at Southeast Missouri Hospital, Cape Girardeau, has
been using the tubes for five or six years and hasn’t heard about
adverse clinical bleeds.
But during the last 18 months, studies conducted in Dr. Hosler’s
laboratory as well as others across the country have revealed two
major problems: APTTs computed from samples drawn with partial-draw
tubes were falsely shortened in up to 15 percent of patients on
unfractionated heparin, and platelet counts tended to be artificially
According to Becton Dickinson, of Franklin Lakes, NJ, the largest
supplier of partial-draw sodium citrate tubes worldwide and sponsor
of the clinical studies, these problems affect only small segments
of the patient population. Decreased APTT occurs with patients on
unfractionated heparin; samples from normal blood donors or patients
on warfarin therapy do not exhibit artificial change in APTT with
partial-draw tubes. And the falsely low platelet count pertains
to the one percent to two percent of patients who have EDTA-associated
platelet clumping, explains Jeffry B. Lawrence, MD, vice president
for medical affairs.
Becton Dickinson has not received complaints from clinician customers
about bleeding as a result of the shortened APTT. "There is enough
noise in the APTT system that clinicians may not notice a bleeding
problem. And if clinicians end up giving 10 percent more heparin
than needed, that may not be enough of an increase for anyone to
pick up an increased rate of bleeding," explains John Brandt, MD,
senior clinical research pathologist, Lilly Research Laboratories,
Indianapolis, and a member of the CAP Coagulation Resource Committee.
Becton Dickinson nevertheless has launched a massive educational
campaign about the drawbacks of the partial-draw tubes, and it will
withdraw the tubes from the market on Sept. 30.
"The potential for leading to inappropriate monitoring of unfractionated
heparin therapy is real," Dr. Brandt says. "If you’re basing your
heparin therapy on samples drawn with these tubes, you may end up
giving more heparin than needed, and the more heparin you give,
the higher the risk of bleeding."
BD first noticed discrepancies in APTT while running a series
of followup tests after correcting a problem with tube stoppers.
The company had discovered that certain lots of stoppers used in
its citrate tubes had begun to release divalent cations into the
citrate solution and, in the process, caused shortening of both
prothrombin time and APTT. After changing to new stoppers, BD found
that the variability in prothrombin time disappeared, as did the
negative bias in APTT with some APTT reagents. However, in initial
studies other reagents still showed some APTT shortening with the
partial-draw tubes. Therefore, the company decided to ask independent
hospital laboratories to study the performance of coagulation tubes
across the entire spectrum of today’s highly sensitive coagulation
The studies were all conducted in the same way, Dr. Lawrence points
out. Phlebotomists in each institution obtained blood samples from
normal blood donors, individuals with various hematological disorders,
and patients receiving unfractionated heparin or warfarin, using
the full-draw 4.5-mL tube and two types of partial-draw tubes-the
2.7 mL and 1.8 mL.
The studies measured clotting time for normal donors and for warfarin
and heparin patients using the PT reagents Thromboplastin C Plus
(Dade), Thromboplastin HS (Sigma), RecombiPlastin (Hemoliance),
and Innovin (Dade), and the APTT reagents ACTIN, ACTIN FS, and ACTIN
FSL (all Dade), FSL (Sigma), Thrombosil I (Hemoliance), SynthASil
(Hemoliance), APTT-C (IL Test), and Platelin (Organon Teknika).
The studies also counted platelets in K3EDTA, sodium citrate, and
CTAD (citrate, theophylline, adenosine, and dipyridamole cocktail)
tubes and tabulated PT and APTT values for each of these tube categories.
The most critical part of these studies, Dr. Lawrence says, determined
the risk each of the three types of tubes posed for misclassifying
heparinized patients. The studies calculated a heparin therapeutic
range using an antifactor Xa chromogenic substrate assay, and for
comparison evaluated the standard therapeutic range of 1.5 to 2.5
times the midpoint of the reference range. The studies then classified
patients as overanticoagulated if their APTT was above the therapeutic
range or underanticoagulated if it fell below.
"We generated for each one of these reagents a number that we call
the misclassification rate, which indicates how many patients would
have been classified one way with the full-draw tube and another
way with the partial-draw tube," Dr. Lawrence explains.
The misclassification risk for the 1.8-mL partial-draw tube ranged
from 6.7 percent with the SynthASil reagent to 14.8 percent with
the ACTIN FSL reagent. The risk for the 2.7-mL tube was 9.3 percent
with ACTIN FSL and 10 percent with the other two reagents.
BD has since learned that the shortened APTT is due to two aspects
of the configuration of the partial-draw tube: a greater amount
of headspace and less vacuum. "There is more space between the top
of the column of blood and the bottom of the stopper in partial-draw
tubes than in standard draw tubes. This increased headspace contributes
to platelet activation, which releases platelet factor 4. Platelet
factor 4 neutralizes heparin in the blood sample, and that leads
to the artificially shortened APTT result," Dr. Lawrence says.
Because there is less vacuum in the partial-draw tubes, it takes
longer for the tubes to fill with blood. "Platelets, as they are
being drawn from the patient, are exposed to a certain shear stress
because of the difference between the pressure in the tube and the
pressure in the patient’s veins," Dr. Lawrence adds. "With the lower
evacuation of the tube, the platelets are exposed to shear stress
for a longer period of time in the partial-draw tubes. So the platelets
become activated, which leads to platelet clumping; the clumped
platelets are not counted by the automated analyzer, and you can
get artificially low platelet counts."
Prothrombin time is not affected, probably because it involves
fewer biochemical reactions than APTT. "With prothrombin time you’re
basically looking only at tissue factor pathway activation, whereas
with APTT, there are many more biochemical steps that can be influenced
by platelet activation," Dr. Lawrence notes.
Dr. Hosler’s laboratory, which serves seven small hospitals and
120 physicians as well as 288-bed Southeast Missouri Hospital, participated
in four studies for BD last year that compared clotting times in
plastic tubes, full-draw tubes, and its in-house partial-draw tubes
and found a wide range of APTT values. In one study, according to
Julie Wengert, head of the blood bank and hematology, "APTTs with
the full-draw tube were in the therapeutic range, but then as you
went down to the 2.7- and 1.8-mL draws, the APTTs dropped down sometimes
20 to 25 seconds. Some patients ran 60 seconds in the full-draw
tubes and 35 or 40 seconds in the 2.7- and 1.8-mL draws."
In another study, the laboratory noticed that CTAD tubes were consistent
across the three draw volumes for heparinized patients. "The 2.7-mL
and 1.8-mL tubes were equivalent to the 4.5-mL draw," Dr. Hosler
says. "But the clotting times decreased significantly when the sodium
citrate tubes were used," he adds.
Now that BD has similar data from studies on more than 400
patients, it is taking a number of steps to get the word out. It
has sent a letter to all its customers worldwide that explains the
variability in platelet counts and in APTT monitoring of patients
on unfractionated heparin.
"Many people have never thought of the coagulation tube as the
source of variability that could be clinically significant," Dr.
Lawrence says. "We want to inform the hematology and clinical communities
that for certain patient populations, the coagulation tube can be
To be sure this information reaches those who use the tubes, BD
is changing the labeling so it warns against using partial-draw
tubes for platelet counting or with samples from heparinized patients.
The company established a Web site (www.bd.com/coag) to provide
another outlet for the information, and it will present data from
its studies at professional society meetings around the world and
in peer-reviewed journals. And at the end of September, it will
discontinue the product.
BD is recommending that laboratories use one of its alternative
products in lieu of the partial-draw tubes, such as its pediatric
or CTAD tubes (see "What laboratories should do," page 50). The
pediatric tubes are smaller than standard 4.5-mL full-draw sodium
citrate tubes and, consequently, will draw lower volumes of blood.
Although the CTAD tube has the same configuration as the partial-draw
tube, its cocktail of theophylline, adenosine, and dipyridamole
platelet inhibitors prevents platelet activation.
The company also hopes to increase awareness of the effects of
variability in the preanalytical phase. "One thing we’ve learned
over the last 20 years is that, as instrumentation has gotten better
and more reliable and more reproducible, and as coagulation reagents
have gotten more sensitive, the preanalytical phase has become more
important as a potential source of error," Dr. Lawrence says.
"Customers have a right and, frankly, an obligation to ask the
vendor of whatever coagulation tube they’re using, ’Does this have
documented clinical performance in the patient population that I’m
studying?’ Laboratories have an obligation to determine that the
specimen containers they’re using for coagulation are free from
bias that potentially could be clinically significant," Dr. Lawrence
Early in his career, Dr. Lawrence, like other laboratory directors,
concentrated on getting the best possible data, achieving the highest
sensitivity and specificity, greatest precision, and greatest accuracy.
"That is what we considered to be our work product," he says.
In hospitals and other health care settings today, laboratories
are redefining their work product to include patient outcome. "Of
course, we want to have improved performance of the APTT and platelet
count, but ultimately that’s not an end in itself. The end we’re
really focusing on is improved anticoagulation therapy and patient
management. That’s where laboratory physicians and other laboratory
professionals can make the biggest contribution to medicine," he
And that is where concepts like the risk of misclassifying heparinized
patients come in. The difference between an effective and a toxic
dose of heparin is narrow. Standard clinical practice for patients
on IV heparin is to calculate the initial dose on the basis of body
weight, then readjust six hours later depending on heparin’s effect
on clotting. "Whether the doctor increases, decreases, or maintains
the current dose of heparin is based on where the APTT of the patient
fits in relation to the therapeutic range. Therefore, the accuracy
of the APTT result is critical for patient management," Dr. Lawrence
Misclassification was defined in terms of the gold standard BD
fulldraw glass tube because published results obtained with that
tube formed the basis of clinical recommendations for treating heparinized
patients. For a coagulation tube to be free of misclassification,
it should yield the same result as the full-draw tube when identifying
patients as falling inside or outside the heparin therapeutic range.
"Based on the clinical studies we’ve done with the products that
will remain on the market-the full-draw tubes, pediatric tubes,
and CTAD tubes-all will be associated with a zero misclassification
rate," Dr. Lawrence says. "So laboratories will be able to use them
with confidence that the results they give to the doctor will be
Karen Sandrick is a freelance writer in Chicago.