Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1109 page)

   Values measured at 37°C must be corrected to the actual temperature of the patient.

   Drugs causing respiratory depression, for example, barbiturates, diazepam, heron, meperidine, and midazolam, cause decrease in pO
₂
.

PARTIAL THROMBOPLASTIN TIME (PTT, aPTT)
*

   Definition

   The PTT assesses the coagulation activity of the intrinsic and common pathways of coagulation. It is the best
screening
test for the diagnosis of disorders of coagulation that do not involve factor VII (extrinsic pathway) or platelet function. The conventional prefix “activated” is obsolete; there is no nonactivated PTT in use. The “activation” reflects a technical aspect of the assay because the reagents contain a negatively charged surface that accelerates the rate of the reaction.

   
Normal range:
22.3–34.0 seconds (varies slightly from lot to lot of reagent, type of the commercial reagent used, and equipment).

   Use

   Screening for hemophilia A and B and other possible coagulopathies (except factors VII and XIII). PTT is not affected by single clotting factor 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 1–2 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
. The inhibitor is commonly a factor VIII inhibitor; LA can also prolong the PTT if LA-sensitive reagents are used.

   Monitoring of therapy with unfractionated heparin. It is not useful in monitoring low molecular weight heparins or fondaparinux; these anticoagulants can be monitored with anti-Xa assay.

   Not recommended for preoperative screening in patients without a personal or immediate family history of unprovoked bleeding.

   Interpretation

Increased ( >36 seconds) In

   Single clotting factor deficiencies, the most common being factor VIII deficiency