PTT or Plasma Thromboplastin Time
The activated partial thromboplastin time (APTT) is a global plasma coagulation test affected by abnormalities in the intrinsic (factors XII, XI, VIII, IX, prekallikrein, and high molecular weight kininogen) and common portions of the classic coagulation pathway. The concept of separate intrinsic and extrinsic pathways of coagulation is useful for understanding and diagnosing blood coagulation abnormalities in vitro, however it should be appreciated that in vivo there are interactions between the two pathways outside of the classic scheme. The APTT will generally be prolonged when a clotting factor level is less than 30-40%. Since the normal range for most clotting factors is 50-150% (and 70-130% for factor XI), a normal APTT does not rule out the possibility of a mild factor deficiency.
The test is performed by adding PTT reagent to plasma and measuring the time for clot formation. PTT reagent contains a particulate activator (kaolin, celite, silica or elagic acid) and a partial thromboplastin, which is phospholipid. The activator initiates the contact system. Then, the remaining steps of the intrinsic pathway take place in the presence of phospholipid.
There are 6 causes of a prolonged APTT (with PT normal or slightly prolonged):
- Pre-analytical errors
- Heparin
- Coagulation factor deficiency associated with risk of hemorrhage
- Coagulation factor deficiency with no risk of hemorrhage
- Lupus anticoagulant
- Specific coagulation factor inhibitor
In the investigation of a prolonged APTT, pre-analytical errors should be ruled out first. The most common pre-analytical cause of a prolonged APTT is contamination with heparin in a sample drawn from an arterial or central line (APTT will often be >200 seconds). The APTT will be affected by an altered plasma to citrate ratio in blue top collection tubes, which may be seen with a high hematocrit (>55%), or a sample with a short or long draw. Other pre-analytical problems include dilution of a sample drawn above an IV, formation of clots in a sample due to inadequate mixing, delay in transport or processing of a sample (>4 hours), and inadequate centrifugation.
The APTT is used most frequently to monitor anticoagulation with unfractionated heparin. Each laboratory should set its own APTT therapeutic range for heparin, corresponding to a heparin level of 0.3-0.7 U/mL. In Saint Luke’s Regional Laboratories this therapeutic range is currently 60-100 seconds. In cases of apparent heparin resistance where the APTT does not rise appropriately, a heparin assay is indicated to determine if a therapeutic amount of heparin is present. Low molecular weight heparin (LMWH) will cause only a mild prolongation of the APTT; if monitoring of LMWH is required, a specific LMWH assay is used. The APTT is prolonged by direct thrombin inhibitors such as Argatroban and lepirudin, and is used to monitor anticoagulation with these drugs. Less frequent causes include hydoxyethyl starch, hematin, Suramin and pegylated drugs.
Hereditary coagulation factor deficiencies which selectively prolong the APTT and are associated with a bleeding tendency include factors VIII, IX and XI. The common acquired coagulopathies such as liver dysfunction and DIC may cause prolongation of the APTT, however the PT will also be prolonged in these disorders, due to multiple clotting factor deficiencies. Hereditary coagulation factor deficiencies which selectively prolong the APTT but are not associated with a risk of hemorrhage include deficiencies of factor XII, prekallikrein and high molecular weight kininogen.
Lupus anticoagulants are acquired inhibitors directed against phospholipid-binding proteins, interfere in vitro with phospholipid-dependent coagulation tests, and are a common cause of APTT prolongation. In vivo, lupus anticoagulants do not interfere with coagulation factor complex formation on the platelet phospholipid surface, and are thus not usually associated with a bleeding tendency.
Approximately 15% of patients with severe factor VIII or IX deficiency develop alloantibodies (inhibitors) to transfused factor concentrate. Autoantibodies against clotting factors may also arise spontaneously, or associated with various diseases and drugs, most commonly directed against factor VIII. The inhibitor-factor complexes are rapidly cleared, resulting in factor deficiency and a severe bleeding tendency.
An approach to the evaluation of a prolonged APTT is outlined in the following algorithm.
The reference range for PTT is 21-34 seconds.
Plasma specimens for PTT must be tested within 4 hours of collection. If this is not possible, the plasma can be stored frozen. Because platelet contamination of plasma can alter the PTT result, specimens should be centrifuged and the plasma separated promptly.
Does a Shortened APTT have any Clinical Significance?
In the realm of coagulation testing, much attention has been paid to the investigation and clinical significance of a prolonged APTT. Little if any attention has been given to the significance of a shortened APTT. There is evidence that increased levels of several coagulation factors (factors VIII, IX, XI, II, and fibrinogen) are independent risk factors for venous thromboembolism (VTE). These factors participate in the coagulation cascade in the intrinsic pathway (factors VIII, IX and XI) and common pathway (factor II and fibrinogen). A shortening of the APTT should reflect increased levels of these factors. Based on this hypothesis, a recent study (Blood, 2004;104: 3631-34) investigated the association between a shortened APTT and the risk of venous thromboembolism (VTE).
A case control study was carried out to compare the APTT values in 605 patients referred for thrombophilia testing after an episode of documented VTE and 1290 healthy controls. The APTT was performed at least 3 months (range 3-79 months) following the thrombotic event, and patients were excluded if they were on anticoagulant drugs, or if they had other conditions potentially affecting the APTT (e.g. lupus anticoagulant, pregnancy). Thrombophilia testing was also performed including factor V Leiden, prothrombin gene mutation, antithrombin, protein C and protein S. The APTT values in patients and controls were expressed as a ratio of the test APTT to a reference (pooled normal plasma) APTT.
The median value for the test/reference APTT ratio was 0.97 in patients and 1.00 in controls – a small difference that was nonetheless statistically significant. The odds ratio for VTE was 2.4-fold increased in patients with an APTT ratio less than 0.87. In Saint Luke’s Regional Laboratories this value would correspond to an APTT of less than 25 sec (our APTT normal range is 22-32 sec). The relative risk of VTE increased as the APTT ratio decreased – the odds ratio for VTE was 4.6 for an APTT ratio of equal to or less than 0.8 (corresponding APTT 23 sec). When patients with the shortest APTT’s were compared with those with the longest APTT’s, the relative risk of VTE was increased 5-fold. These odds ratios have been adjusted for age, sex, and the presence of inherited thrombophilic disorders. Factor VIII assays were performed on a subgroup of the samples. When the results were adjusted for factor VIII levels, the increased relative risk of VTE remained significant, indicating that high levels of factor VIII were not the only determinants of the shortened APTT’s.
In summary, this study suggests that a shortened APTT may be associated with an increased risk of VTE, independent of inherited thrombophilic defects. A direct causal effect of the hypercoagulability reflected by the shortened APTT cannot be established. If the results of this study are confirmed and extended by others using a variety of different reagents, the APTT may prove to be a simple, cheap and useful screening test for high coagulation factor levels in the investigation of thrombophilia.
