Activated Partial Thromboplastin Time
Activated Partial
Thromboplastin Time
1
Roger S. Riley, M.D., Ph.D.
April, 2005
Feature
Synonyms
aPTT, APTT.
Test Description
The aPTT is functional determination of the intrinsic pathway of coagulation (factors
XII, XI, IX, VIII, V, II, I, prekallikrein, high molecular weight kininogen). This pathway
is intitated by the interaction of Factor XII with a negatively charged surface. A cascade mechanism results in fibrin production and clot formation. The aPTT is utilized to
detect congenital and qcquired abnormalities of the intrinsic coagulation pathway and
to monitor patients receiving heparin.
Patient
Preparation
No specific patient preparation is required. However, since lipemia may interfere with
photo-electric measurements of clot formation, specimens should not be obtained after a meal. In patients receiving intermittent heparin injections, peripheral blood for
aPTT analysis should be obtained one hour before the next dose of heparin is scheduled. The specimen should not be drawn from an arm with a heparinized catheter or
heparin lock.
Specimen
Citrated, platelet-poor plasma is used for the aPTT.
Specimen
Collection
and
Preparation
Specimen requirements for coagulation assays are described in NCCLS H21-A3 (Collection, Transport, and Processing of Blood Specimens for Coagulation Assays; Approved Guideline - Third Edition, December, 1998). These requirements are summarized below.
Citrated, platelet-poor plasma is prepared from peripheral venous blood collected by
clean, nontraumatic venipuncture directly into a plastic or siliconized glass tube containing 109 nM (3.2%) trisodium citrate at a ratio of 9:1. With a blood collection set,
the tube for coagulation analysis automatically fills to the correct volume. The tube
should be immediately inverted at least four time after filling. A needle with a gauge
of 22 to 19 should be utilized in adult patients, while a 21 to 23 gauge needle is suitable in pediatric patients. A traumatic venipuncture can activate coagulation factors,
leading to a shortened aPTT. In patients receiving heparin, extreme care must be
taken to avoid release of platelet factor 4 (PF4), which is a potent heparin inhibitor.
Syringe draws - Blood obtained from a syringe draw is not preferred for aPTT analysis
due to safety issues and the increased chance of specimen hemolysis or clotting. If a
syringe must be used for aPTT specimen collection, a small volume syringe (< 20
mL) is recommended. The double syringe technique is recommended, with the second tube used for coagulation alalysis. The blood must be transferred from the syringe to a plastic or siliconized glass tube tube containing the proper amount of anticoagulant within one minute after collection.
Indwelling catheters - Blood dilution and contamination with heparin are risks when
blood specimens collected from an indwelling catheter are utilized for the aPTT. If
such a specimen must be used, the line should first be flushed with saline, and the
first 5 mL or six dead space volumes of the catheter drawn and discarded. Care must
also be taken free the catheter and blood collection system from air leaks.
Activated Partial Thromboplastin Time
Specimen
Collection and
Preparation
(Cont’D)
2
Multiple specimens - If multiple blood specimens are obtained, the aPTT should
be performed on the second or third tube. If the blood is being obtained only
for coagulation testing, the first tube should be drawn and discarded, and the
second tube submitted to the laboratory.
Specimen preparation - Platelet-poor plasma (platelet count < 10 x 109/L) is prepared from the whole blood specimen by centrifuging the capped specimen tube at
an appropriate speed for an appropriate time. Centrifugation at 1500 g for 15 minutes at room temperature is recommended for this purpose. A centrifuge with a
swing-out bucket rotor should be used to avoid remixing of platelets and plasma during plasma removal.
Specimen storage Unheparinized specimens - Specimens for aPTT analysis from unheparinized
patients should be maintained centrifuged or uncentrifuged with plasma remaining on top of the cells in an unopened tube maintained at 2-4oC or 1824oC and tested within four hours of specimen collection.
Heparinized specimens - Specimens suspected of containing heparin should
be centrifuged within one hour of collection and the plasma analyzed within
four hours of collection. Plasma should be separated and removed within one
hour if the specimen is being transported to a remote location for analysis or
otherwise agitated.
Frozen plasma - Plasma should be separated from specimens which cannot be
analyzed within four hours and frozen at -20oC for up to two weeks or at
-70oC for up to six months in a frost-free freezer. Frozen plasma specimens
should be rapidly thawed at 37oC, gently mixed, and analyzed immediately or
stored at 4oC for a maximum of two hours prior to analysis.
Causes for Specimen Rejection - The aPTT cannot be performed on speciments that
are clotted, visibly hemolyzed, collected into the wrong anticoagulant, collected into
an improper quantity of anticoagulant, not labeled, or improperly labeled.
Test
Methodology
The aPTT is usually performed by automated testing in the batch or stat mode. In
the aPTT an aliquot of undiluted, platelet-poor plasma is incubated at 37oC with a
particulate factor XII activator (i.e., silica, celite, kaolin, micronized silica, ellagic
acid, etc.). A reagent containing phospholipid (partial thromboplastin) is added, followed by CaCl2. The time required for clot formation is measured by one of a variety
of techniques (photo-optical, electromechanical, etc.). The aPTT result is reported as
the time required for clot formation after the addition of CaCl2.
Many different phosophlipid reagents animal and plant origin, such as cephalin, have
been used as partial thromboplastins, and a variety of activating substances are in
use. The sensitivity of the assay to factor deficiencies, inhibitors, and heparin varies
with the reagents used in the assay.
Normal Values and
Critical Limits
24 - 37 sec (Jordan, C.D. et al., Normal reference laboratory values. N. Engl. J. Med.
327:718-724, 1992). Statistically, the aPTT is slightly lengthened in young individuals and slightly shortened in older populations. Premature infants have prolonged
aPTT values which return to normal by 6 months of age. However, age-specific normal ranges are not utilized in patient care at this time.
Interferences
Lipemia and hyperbilirubinemia interfere with the detection of clot formation by
photo-optical methods. The results of the aPTT may be affected by a wide variety of
factors, including the manner of blood coagulation, the type of container, the type of
anticoagulant, specimen transport and storage conditions, incubation time and temperature, assay reagents, and the method of end point detection.
Activated Partial Thromboplastin Time
Clinical Utilization
3
The PT and aPTT are the fundamental assays of the coagulation system. The principal clinical uses of the aPTT include: (1) the detection of hereditary or acquired deficiencies or defects of the intrinsic and common pathway coagulation factors (factors
XII, XI, IX, VIII, prekallikrein, high molecular weight kininogen), (2) monitoring
heparin anticoagulant therapy, (3) the detection of coagulation inhibitors (i.e., lupus
anticoagulant), and (4) to monitor coagulation factor replacement therapy in patients
with hemophilia.
The aPTT is increased above the upper limit of normal with hereditary or acquired
intrinsic factor deficiencies < 40% (factor VIII:C, Factor IX, Factor XI, Factor XII,
vWf), lupus anticoagulants, or specific inhibitors of the intrinsic coagulation factors.
Other causes of an elevated aPTT include liver disease, disseminated intravascular
coagulation (DIC), heparin or anticoagulant therapy, or improper specimen collection
(i.e., traumatic phlebetomy or hemolyzed specimen).
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