PARTIAL THROMBOPLASTIN TIME
(PTT, aPTT)

Sample type:

Blood sample (citrate)

Uses:

• Screening for hemophilia A and B and other possible coagulopathies (except factors VII and XIII). PTT does not detect single clotting factors defects above 40% of normal.

• Detection of clotting inhibitors. This is best performed by mixing studies once an otherwise unexplained prolonged PTT is found. Mixing equal parts of patient and normal plasma (1:1) for 1-hggggyyy2 hours at 37°C normalizes the prolonged PTT if it is caused by a coagulation factor deficiency but not if it is caused by an inhibitor. This inhibitor is most commonly factor VIII inhibitor, or it can be LA if sensitive reagents are used).

Precaution:

None; however, a high-fat meal prior to the blood draw may interfere with the test and should be avoided.

Interfering factor:

• See appendix J for drug that affect test outcome.

• Hemolized plasma shortens APTT in normal patient but not in abnormal (heparinized) patient.

• Very increase or decrease Hct

• Incorrect ratio of blood to citrate

Pre-analytical errors:

• the ratio of patient plasma and normal plasma

• the source and buffering status of the normal plasma

• the conditions of incubation

Post-analytical errors:

• the principle used in the interpretation of the results

• Check instrument settings

Reference range:

22.3-34.0 seconds