General Approach in Investigation of - PowerPoint Presentation

Slide1

General Approach in Investigation of Hemostasis

Lecture 4 :

Activated Partial Thromboplastin Time (aPTT)

The Islamic University – Gaza

Faculty of Health Science

Medical Technology DepartmentSlide2

Also known as: Partial Thromboplastin Time (PTT); Kaolin Cephalin Clotting Time (KCCT).

Is a performance indicator measuring the efficacy of both the "intrinsic" (the contact activation pathway) and the common coagulation pathways.APTT is the most widely used method for monitoring intravenous heparin anticoagulant therapy (Heparin acts to accelerate

Antithrombin which inhibits the actions of thrombin).Activated Partial Thromboplastin Time (aPTT)Slide3

The PTT evaluates the coagulation factors XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight

kininogen (HK).It measures deficiencies mainly in factors VIII, IX, XI, and XII, but can detect deficiencies of all factors except III (tissue Factor) and VII.

The PTT can also detect Nonspecific inhibitors, such as lupus anticoagulant and anticardiolipin antibodies, which associated with clotting episodes and with recurrent miscarriages, especially those that occur in the second or third trimester. Slide4
Slide5

Unexplained bleeding or bruising(The PTT may be ordered along with other tests such as a PT)A thromboembolism like DIC or Liver Disease.Recurrent miscarriages or a thrombotic episode , the PTT may be ordered as part of an evaluation for lupus anticoagulant or

anticardiolipin antibodies.A person is on intravenous (IV) or injection heparin therapy (Monitoring Therapy).

(the Therapeutic range should be 1½ to 2½ times normal).When is it ordered?Slide6

When someone is switched from heparin therapy to longer-term warfarin (COUMADIN®) therapy, the two are overlapped and both the PTT and PT are monitored until the person has stabilized.Pre-surgical evaluation for bleeding tendencies.Slide7

Clinical Significance

When the partial

thromboplastin time is used in combination with the prothrombin time, most procoagulant disorders can be classified.Slide8

Patient platelet poor plasma (PPP) is incubated at 37°C then phospholipid (cephalin) and a contact activator (e.g. Kaolin) are added followed by the calcium (all pre-warmed to 37°C). Addition of calcium initiates clotting and timing begins for a fibrin clot to form.

Kaolin is used as a surface activator. It binds directly to FXII resulting in its activation to XIIa. XIIa cleaves FXI to XIa

but in the absence of calcium, activation of the subsequent factors does not occur. PrincipleSlide9

Cephalin is a phospholipid substitute that replaces platelet phospholipid in the test (remember the test uses platelet poor plasma and so requires a source of phospholipid for coagulation to occur.)Slide10
Slide11

Pre- warm a sufficient quantity of calcium chloride reagent to 37oC for at least 10 min.Pipet 100 µL of Sample into a labeled test tube, incubate 1-2 min.

Into each test tube, add 100 µL of partial thromboplastin reagent.Incubate the mixture at 37oC for a minimum of three minutes. (optimum activation of contact factors)

Forcefully add in100 µL of calcium chloride into mixture and start the stop watch immediately.ProcedureSlide12

Mix the tube once, immediately after adding the calcium reagent. Allow the tube to remain in the water bath, approximately 20 seconds, mixing occasionally.After 20 seconds, remove the tube from the water bath/heat block. Wipe off the outside of the tube. Gently tilt the tube back and forth until a visible clot forms. Stop the stop watch immediately and record the time in seconds.

Carry out 1 significant figure passed the decimal point. For example, if your result is 30.31, report as 30.3 seconds.

The result must be run in duplicate.Repeat the steps using Normal Control.Slide13

Reference Range: 27-35 seconds.

Critical Value: > 70 seconds Reference RangeSlide14

Associated with specimen (Preanalytical)

Inappropropriate ratio of anticoagulant to blood

Failure to correct citrate volume if hematocrit > 55%Clotted, hemolyzed or lipemic samplesLack of PPPDelay in testing or processingInappropriate storage

Sources of ErrorSlide15

Associated with Reagent (Analytical)Incorrect preparation of reagents

Use of reagents beyond reconstituted stability time or expiration dateContaminated reagent.

Associated with procedure (Analytical)Incorrect temperatureIncorrect incubation timesIncorrect volumes of sample, reagents or bothSources of ErrorSlide16

CommentsThe activator for the APTT e.g. kaolin, silica etc

should be selected to ensure that the reaction is sensitive to mild deficiencies of factors VIII, IX and XI (i.e. levels of 0.35-0.4 IU/ml).If an optical density method is used to monitor clot formation, falsely normal APTTs may occur if the patient’s plasma is turbid (e.g. hyperbilirubinaemia,

hyperlipidaemia).When selecting a phospholipid for the APTT it is important to choose a reagent that is sensitive to deficiencies in clotting factors. Some laboratories may choose APTT reagents that are insensitive to a Lupus Anticoagulant to prevent it (if present) in interfering with factor assays. In such cases, an alternative reagent may be necessary if the APTT is used as a screening test for a lupus anticoagulant.Slide17

A deficiency of factor XIII does not prolong the APTT or the PT.The APTT can be made more or less sensitive to specific clotting factors by varying the incubation time. A short incubation time e.g. 2 minutes makes the test very sensitive to the levels of contact factors whereas a long incubation period make it very insensitive to these factors. Many laboratories employ a 5 minute incubation period. If a contact factor deficiency is suspected, then comparing a short and long incubation time may be useful.Slide18

In patients receiving very high concentrations of unfractionated heparin e.g. during cardiopulmonary bypass, the APTT will be unclottable. In these cases, monitoring of the degree of heparinisation

is undertaken using a different assay - usually the Activated Clotting Time (ACT). New anticoagulants are used now Ribaroxaban and DabigatranSlide19