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PT/PTT are laboratory tests that measure the
clotting time (how long it takes blood to clot.)
PT/PTT are blood tests and
INR is a
ratio calculated from the
PT.
At least a dozen blood proteins, or blood clotting factors, are needed to clot blood and stop bleeding (coagulation).
Prothrombin, or factor II, is one of several clotting factors produced by the liver. Prothrombin time (PT) measures the presence and activity of five different blood clotting factors (factors I, II, V, VII, and X). This test measures the integrity of the extrinsic and common pathways of coagulation. The prothrombin time (PT) measures the clotting time from the activation of factor VII through the formation of the fibrin clot. Adequate amounts of vitamin K are needed to produce prothrombin. Warfarin (Coumadin) is sometimes prescribed as a "blood thinner" because it is an effective vitamin K antagonist (blocks the formation of vitamin K).
The activated partial thromboplastin time (PTT) measures the function of several other clotting factors, found in the intrinsic and common blood clotting pathways. It measures the clotting time from the activation of factor XII through the formation of the fibrin clot. PTT prolongations are caused by factor deficiencies (factors VIII, IX, XI, and/or XII), or inhibitors (most commonly, heparin). Heparin prevents clotting by blocking certain factors in the intrinsic pathway.
PTT and activated partial thromboplastin time (aPTT) are basically the same thing. The aPTT has largely replaced the older PTT, which was unable to incorporate variables in surface/contact time:
http://www.rnceus.com/coag/coagptt.html
PT is used to monitor treatment with warfarin (Coumadin). Once warfarin is discontinued, it usually takes several days to clear it from the system. The PTT test allows the provider to check that there is enough heparin in the blood to prevent clotting, but not so much as to cause bleeding. Once heparin is discontinued, its blood-thinning effects usually only last a few hours.
The greater the PT/PTT values, the longer it takes the blood to clot. HIGH PT/PTT = risk for BLEEDING. LOW PT/PTT = risk for blood clots/ stroke.
The INR stands for International Normalized Ratio. The INR is calculated from the PT and is intended to allow valid comparisons of results regardless of the type of PT reagent used among different laboratories (INR = [patient PT / mean normal PT]). The INR is a method of standardizing the PT for coumadin anticoagulation. Before the INR, different labs using different reagents had different controls and widely differing PT value ranges.
An INR of 1 means the blood clots "normally" for that pt. The greater the INR, the longer it takes the blood to clot.
The warfarin (Coumadin) dosage for people being treated to prevent the formation of blood clots is usually adjusted so that the prothrombin time is about 1.5 to 2.5 times the normal value (or INR values 2 to 3). A patient may take warfarin to anticoagulate for atrial fibrillation (target INR around 2.5) or for a mechanical heart valve (target INR 3). An INR of 3 means the blood takes about three times as long to clot compared with the normal value for that pt.
Which value, PT or PTT, does heparin influence?
Which one does coumadin?
Stumped?
You can find the right answer by counting to 10:
- - - - - - - - - - = 10
H E P A R I N (7 letters) + 3 (PTT) = 10
C O U M A D I N (8 letters) + 2 (PT) = 10
Here's another one:
What is the antidote for heparin overdose?
Protamine sulfate (just remember P M S)
What is the antidote for too much Coumadin?
Vitamin K (just remember the hard "C" at the beginning of coumadin!)
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