Coagulation Case Study
John D. Olson, MD, PhD
This is the fourth in a periodic series of articles written by
members of the CAP Coagulation Resource Committee and focusing on
laboratory evaluation of coagulation disorders
history. A 42-year-old woman who was the victim of a motor vehicle
accident presented to a hospital emergency room. She was conscious
at the time of presentation. Following an immediate imaging evaluation,
she was taken to the operating room for open reduction of multiple
fractures of the left hip and femur. She received four units of
packed red blood cells intraoperatively.
During the postoperative period, after her admission to the surgical
intensive care unit, the patient’s vital signs were stable and laboratory
test results were obtain-ed (Table 1). The laboratory findings
led to the evaluation of the patient's prolonged activated partial
At the beginning of the algorithm for determining the etiology
of an abnormal aPTT at the hospital, the thrombin clotting time
was longer than 150 seconds. This raised the possibility that the
specimen was contaminated by heparin. Consultation with the nursing
staff in the intensive care unit confirmed that the specimen had
been drawn from an arterial line that was “kept open” with
heparin. Repeat coagulation studies performed on a specimen collected
from a peripheral vein revealed an aPTT within the reference range.
This brief clinical history provides an introduction to a laboratory
evaluation of a prolonged aPTT. The aforementioned clinical situation
is relatively common. Despite efforts to flush heparin from central
venous lines, heparin contamination of coagulation specimens drawn
from such lines continues to be a problem.
Since it was first described in 1953, the partial thromboplastin
time has been a valuable tool for monitoring heparin therapy and
evaluating patients who have a suspected bleeding disorder. Methods
for automating the reading of the end point for the prothrombin
time and aPTT were developed in the 1960s and early 1970s. At the
same time, manufacturers began to provide reagents for performing
the tests. In the 1970s, the PTT method changed. Kaolin, celite,
micronized silica, or elagic acid were added to the test to activate
contact factors and change the PTT to aPTT. This is the method that
prevails today. The responsiveness of the reagents used for the
aPTT continue to vary widely in all clinical uses.
Before addressing clinical etiologies of a prolonged aPTT, it is
important to note that several preanalytical (spurious) factors
can cause an abnormal time. Among them are an unusually high or
low hematocrit causing an alteration in the concentration of citrate,
con-tam-ination of the specimen with EDTA, and formation of clots
in the specimen. One must be able to exclude these and other spurious
causes when evaluating any coagulation test.
Prolongation of the aPPT typically can be attributed to five common
causes (Table 2). Each of the different etiologic categories
for a prolonged aPTT requires a specific action on the part of the
clinician who responds to the abnormal test result. The following
are of clinical significance:
1. Medication aPTT is the test most often used
for monitoring therapy with unfractionated heparin, which can contaminate
specimens for coagulation testing that are drawn inappropriately
from arterial or central venous lines. When used to monitor heparin
therapy, the aPTT can be of value in terms of adjusting dosage.
Contamination of a specimen by heparin can lead to unnecessary and
costly additional laboratory work and, if not detected promptly,
to inappropriate testing. Furthermore, it can obfuscate the interpretation
of those tests.
Although heparin is the medication most likely to prolong the aPTT,
new medications, such as hirudin analogues and argatroban, are beginning
to be used in clinical situations and can inhibit thrombin and produce
a prolonged aPTT. Other less frequent causes of such prolongation
include the presence of hydroxy-ethyl starch, hematin, and Suramin.
In addition, Taularidine, an additive in some intravenous medications,
can lengthen the aPTT.
It is important to exclude early a medication etiology for a prolonged
aPTT. This may require using the thrombin clotting time or adding
heparinase to the specimen to interpret the result correctly.
2. Coagulation factor deficiencies associated with significant
hemorrhageInherited deficiencies of not only the most
commonly deficient coagulation factorsfactors VIII, IX, and
XIbut also other coagulation factors can significantly prolong
the aPTT. In addition, acquired deficiencies of individual or multiple
coagulation factors, as might be seen in patients with liver disease
or consumption coagulopathy, can prolong an aPTT. The latter are
frequently accompanied by a prolonged PT. When coagulation factors
are severely deficient, a clinician often must replace those deficient
factors or, in the case of acquired deficiencies, treat the underlying
etiology of the factor deficiency.
3. Coagulation factor deficiencies of little or no clinical
significanceMost medical students are taught about the
intrinsic and extrinsic activation of the common pathway of coagulation.
This approach to thinking about the mechanism of thrombin generation
has emanated from our knowledge about the activation processes that
occur in vitro.
Activation of coagulation in vivo proceeds differently and involves
considerable cross activation between the intrinsic and extrinsic
systems. Because of this difference between the in vivo and in vitro
activation processes, patients may have significant deficiencies
of coagulation factors involved in contact activation (factor XII,
prekallikrein, and high-molecular-weight kininogen) that lead to
a markedly prolonged aPTT that has no associated bleeding diathesis.
Such deficiencies generally are of no consequence.
There have been infrequent reports of patients with severe deficiencies
of contact factors who have hypercoagulability. This is presumed
to be the consequence of these contact factors activating fibrinolysis,
which, when reduced, can lead to hypercoagulability. In general,
a prolonged aPTT caused by deficiency of factor XII, prekallikrein,
or high-molecular-weight kininogen is a nuisance and of no clinical
4. Nonspecific (lupus-type) anticoagulant (LA)A
previous coagulation case study detailed the evaluation of antiphospholipid
antibodies and LAs. (See “Diagnosing antiphospholipid antibody
syndrome”, CAP TODAY, March 1999, page 84.) Lupus anticoagulants
can significantly prolong the aPTT that usually fails to correct
in a mixing study. Demonstrating that this anticoagulant activity
is dependent on phospholipid confirms the diagnosis of an LA. The
patient who has an LA should be evaluated for a hypercoagulable
state and a propensity for thrombosis. If an LA is found in a patient
with thrombosis, it may be necessary to prolong, and possibly increase,
the intensity of anticoagulation therapy.
5. Specific coagulation factor inhibitorApproximately
15 percent of patients who have severe factor VIII or severe factor
IX deficiency develop alloantibodies that recognize the deficient
factor when it is transfused. Furthermore, patients may spontaneously
develop autoantibodies that recognize and rapidly clear their own
coagulation factors. The vast majority of these specific autoantibodies
that lead to profound hemorrhage are directed at the factor VIII
molecule. It is critical that clinicians be able to recognize such
an inhibitor because the hemorrhage may be life-threatening and
it may be extremely difficult to manage the patient.
Laboratory test-based algorithm. It is important to evaluate
a new reagent or an instrument used to perform the aPTT in order
to determine the effectiveness of the method in identifying the
aforementioned etiologies. The validation of a new aPTT method will
require: determining its sensitivity to heparin and, possibly, other
drugs, and its sensitivity to a factor deficiency, particularly
factors VIII and IX; determining how the aPTT reagent performs in
evaluating the mixing study in factor-deficient plasmas and plasmas
containing anticoagulant; and determining the sensitivity of the
reagent to an LA.
An algorithm for evaluating a prolonged aPTT appears in Fig.
1. It is not the only clinically acceptable algorithm, but it
is one approach that may be helpful in sorting the five potential
clinical outcomes. The algorithm is described briefly below.
In most cases, laboratory professionals will know the value of
the fibrinogen by the time they evaluate a prolonged aPTT. If this
value is not known, we find it useful, in our laboratory, to determine
that a fibrinogen concentration is sufficient to support the test.
(It is frustrating to fully evaluate a prolonged aPTT only to find
that the patient suffers from hypofibrinogenemia.) If the fibrinogen
concentration is sufficient, it is useful to conduct an evaluation
to exclude heparin or other medications.
Our laboratory has found that the use of a sensitive thrombin time
(control of 18 to 20 seconds) can be helpful. With most methods,
if there is sufficient heparin in the specimen to prolong the aPTT
beyond the reference range by only one second, the thrombin time
will be longer than 150 seconds. Therefore, the finding of a measurable
thrombin time excludes heparin as the etiology of a significantly
prolonged aPTT. A mixing study is useful if the thrombin time is
A recent CAP proficiency testing questionnaire queried a large
number of laboratories about method and reagent selections for PT
and aPTT mixing studies. The results of that query demonstrated
wide variability in approaches to performing the mixing study. A
manuscript that summarizes the findings is being prepared. It will
provide recommendations for performing the mixing study. A few points
are worth addressing here:
- Mixing studies on very mildly pro-longed aPTTs (less than three
to five seconds above the reference range) often will be confusing
and difficult to interpret.
- The normal plasma used for mixing with the patient's plasma
should have a known concentration of coagulation factors, particularly
factors V and VIII (near 100 percent), and should be a fresh-frozen
and platelet-depleted specimen.
- The mixing study primarily is of value in determining whether
an inhibitor is present. It can be very difficult to interpret
the control result and the result of test assays for time dependency
of known inhibitors.
The approach to the mixing study described herein involves mixing
an equal volume of the patient’s plasma and the control plasma and
immediately performing the aPTT. If the resulting aPTT fails to
correct to the reference range, an inhibitor likely is present and
evaluation of the type (cause) of the inhibitor is indicated. If
the value corrects into the reference range, one must perform an
incubated mix to determine if a time-dependent inhibitor is present.
Performing an incubated mix (Fig. 2) involves mixing an
equal volume of the patient’s plasma and the pooled normal plasma
and incubating the mixed specimen (tube A). This becomes
the test specimen. To form the control specimen, the patient specimen
(tube B) and the pooled normal plasma (tube C) are
incubated for the same time (one to two hours; 37 ° C). After incubation,
equal parts of tubes B and C are mixed (tube D).
At the end of the incubation, a laboratory professional measures
the aPTT of the mixed specimens (tube A, the test, and tube
D, the control). If tubes A and D produce the
same aPTTs, an interpretation of a corrected mixing study is made
and an evaluation for factor deficiency would be indicated.
The patient's clinical history will determine whether one initially
would evaluate contact activation factors or procoagulant factors.
If the test specimen is prolonged when compared with the control
specimen, one can interpret that the mix failed to correct and an
inhibitor is present. The next step is to determine whether the
inhibitor is phospholipid-dependent. A variety of tests are available
to determine phospholipid dependence of inhibition, and more than
one of these should be available for the evaluation because none
are sufficiently sensitive or specific to the presence of an LA.
If phospholipid dependence is determined, and the patient's clinical
history is consistent with this determination, the diagnosis is
an LA. If phospholipid dependence is not found, the patient should
be evaluated for the presence of a specific coagulation inhibitor.
Conclusion. Although simplified, this algorithm offers an
approach to evaluating a prolonged aPTT. As one performs the evaluation,
it is important to consider the following:
- the availability of other medications, in addition to unfractionated
heparin, that can prolong the aPTTl
- the nuances of the mixing study and its interpretation
- the difficulties of assigning the phospholipid dependence of
- the critical importance of interpreting the laboratory result
in view of the patient’s clinical history and findings.
Mixing studies are as much an art form as a science and must be
interpreted by laboratory professionals who have intimate knowledge
of the test performance in their laboratories.
Dr. Olson is professor and director of Clinical Laboratories,
Department of Pathology, University of Texas Health Science Center,
San Antonio. He is chair of the CAP Coagulation Resource Committee.