FACTOID # 3: South Carolina has the highest rate of violent crimes and aggravated assaults per capita among US states.
 
 Home   Encyclopedia   Statistics   States A-Z   Flags   Maps   FAQ   About 
   
 
WHAT'S NEW
 

SEARCH ALL

FACTS & STATISTICS    Advanced view

Search encyclopedia, statistics and forums:

 

 

(* = Graphable)

 

 


Encyclopedia > Prothrombin time

The prothrombin time (PT) and its derived measures of prothrombin ratio (PR) and international normalized ratio (INR) are measures of the extrinsic pathway of coagulation. They are used to determine the clotting tendency of blood, in the measure of warfarin dosage, liver damage and vitamin K status. The reference range for prothrombin time is usually around 12-15 seconds; the normal range for the INR is 0.8-1.2. It is used in conjunction with the activated partial thromboplastin time (aPTT) which measures the intrinsic pathway. The coagulation of blood is a complex process during which blood forms solid clots. ... Warfarin (also known under the brand names of Coumadin®, Jantoven®, Marevan®, and Waran®) is an anticoagulant medication that is administered orally or, very rarely, by injection. ... Vitamin K denotes a group of 2-methilo-naphthoquinone derivatives. ... In health-related fields, a reference range is a set of values of some measurement that a physician or other health professional can use to interpret a set of results for a particular patient. ... The partial thromboplastin time (PTT) or activated partial thromboplastin time (aPTT) is a performance indicator measuring the efficacy of both the intrinsic and the common coagulation pathways. ...

Contents

Laboratory measurement

Methodology

The prothrombin time can be measured roughly on whole blood (which is done in neonates), but is more commonly measured from blood plasma. Blood is drawn into a test tube containing liquid citrate. Citrate acts as an anticoagulant by binding the calcium in a sample. The blood is mixed, then centrifuged to separate blood cells from plasma. A human infant The word Infant derives from the Latin in-fans, meaning unable to speak. ... Blood plasma is the liquid component of blood, in which the blood cells are suspended. ... A test tube (Sometimes culture tube) is a kind of laboratory glassware, composed of a fingerlike length of glass tubing, open at the top, sometimes with a rounded lip at the top, and a rounded U shaped bottom. ... Chemical strucutre of citric acid. ...


The plasma is analyzed by a Medical Technologist on an automated instrument, which takes a sample of the plasma. An excess of calcium is added (thereby reversing the effects of citrate), which enables the blood to clot again. For an accurate measurement the proportion of blood to citrate needs to be fixed; many laboratories will not perform the assay if the tube is underfilled and contains a relatively high concentration of citrate. This is because vacutainer test tubes generally contain a powdered anticoagulant to prevent blood from clotting. For the prothrombin time test the appropriate sample is the blue top tube, or citrate tube, which is a liquid anticoagulant. Just as adding water to any solution will dilute it, adding liquid anticoagulant to blood will dilute it. This dilution will cause a falsely long prothrombin time. So, all analysis takes this dilution into account by multiplying the result by 1.1 to account for the dilution. If a tube is underfilled or overfilled with blood, the standardized dilution of 1.1 is no longer valid. A Medical Technologist (MT) is an Allied Health Professional who does clinical analysis on human body fluids and performs other types of medical testing. ... A range of Vacutainer tubes containing blood. ...


Tissue factor (also known as factor III or thromboplastin) is added, and the time the sample takes to clot is measured optically. Thromboplastin is a substance present in tissues, platelets, and leukocytes necessary for the coagulation of blood; in the presence of calcium ions thromboplastin is necessary for the conversion of prothrombin to thrombin, an important step in coagulation of blood. ...


The prothrombin ratio is the prothrombin time for a patient, divided by the result for control plasma.


International normalized ratio

Because of differences between different batches and manufacturers of tissue factor (it is a biologically obtained product), the INR was devised to standardise the results.


Each manufacturer gives an ISI (International Sensitivity Index) for any tissue factor they make. The ISI value indicates how the particular batch of tissue factor compares to an internationally standardized sample. The ISI is usually between 1.0 and 1.4.


The INR is the ratio of a patient's prothrombin time to a normal (control) sample, raised to the power of the ISI value for the control sample used.



Interpretation

The prothrombin time is the time it takes plasma to clot after addition of tissue factor (obtained from animals). This measures the quality of the extrinsic pathway (as well as the common pathway) of coagulation. Blood plasma is the liquid component of blood, in which the blood cells are suspended. ... Thromboplastin is a substance present in tissues, platelets, and leukocytes necessary for the coagulation of blood; in the presence of calcium ions thromboplastin is necessary for the conversion of prothrombin to thrombin, an important step in coagulation of blood. ... The coagulation of blood is a complex process during which blood forms solid clots. ...


The speed of the extrinsic pathway is greatly affected by levels of factor VII in the body. Factor VII has a short half-life and its synthesis requires vitamin K. The prothrombin time can be prolonged as a result of deficiencies in vitamin K, which can be caused by warfarin, malabsorption or lack of intestinal colonization by bacteria (such as in newborns). In addition, poor factor VII synthesis (due to liver disease) or increased consumption (in disseminated intravascular coagulation) may prolong the PT. Factor VII (old name proconvertin) is one of the central proteins in the coagulation cascade. ... Half-Life For a quantity subject to exponential decay, the half-life is the time required for the quantity to fall to half of its initial value. ... Vitamin K denotes a group of 2-methilo-naphthoquinone derivatives. ... Warfarin (also known under the brand names of Coumadin®, Jantoven®, Marevan®, and Waran®) is an anticoagulant medication that is administered orally or, very rarely, by injection. ... Malabsorption is the state of impaired absorption of nutrients in the small intestine. ... A human infant The word Infant derives from the Latin in-fans, meaning unable to speak. ... The liver is an organ in vertebrates including humans. ... Disseminated intravascular coagulation (DIC) is a pathological process in the body where the blood starts to coagulate throughout the whole body. ...


Factors determining accuracy

Lupus anticoagulant, a circulating inhibitor predisposing for thrombosis, may skew PT results, depending on the assay used.[1] Variations between various thromboplastin preparations have in the past led to decreased accuracy of INR readings, and a 2005 study suggested that despite international calibration efforts (by INR) there were still statistically significant differences between various kits[2], casting doubt on the long-term tenability of PT/INR as a measure for anticoagulant therapy.[3] Lupus anticoagulant is an autoimmune disorder caused by antibodies that bind to phospholipids and proteins associated with the cell membrane. ...


Statistics

An estimated 800 million PT/INR assays are performed annually worldwide.[3]


Near-patient testing

In addition to the laboratory method outlined above, near-patient testing (NPT) is becoming increasingly common in some countries. In the United Kingdom, for example, near-patient testing is used both by patients at home, and by some anticoagulation clinics (often hospital-based) as a fast and convenient alternative to the lab method. After a period of doubt about the accuracy of NPT results, a new generation of machines and reagents seems to gaining acceptance for its ability to deliver results close in accuracy to those of the lab.[4]


In a typical NPT setup a small table-top device is used; for example the Roche Coagucheck® S, or the more recently (2005) introduced HemoSense INRatio®. A drop of capillary blood is obtained with an automated finger-prick, which is almost painless. This drop is placed on a disposable test strip with which the machine has been prepared. The resulting INR comes up on the display a few seconds later. Similar testing methods are used by diabetics on insulin, and are easily taught and practiced. Hoffmann-La Roche, Ltd. ... For the disease characterized by excretion of large amounts of severely diluted urine, see diabetes insipidus. ... It has been suggested that Oral insulin be merged into this article or section. ...


Local policy determines whether the patient or a coagulation specialist (nurse, general practitioner or hospital doctor) interprets the result and determines the dose of medication. In Germany, patients may adjust the medication dose themselves[citation needed], while in the UK and the USA this remains in the hands of a health care professional.


The advantages of the NPT approach are obvious: it is fast and convenient, usually less painful, and offers, in home use, the ability for patients to measure their own INRs when required. Among its problems are that quite a steady hand is needed to deliver the blood to the exact spot, that some patients find the finger-pricking difficult, and that the cost of the test strips must also be taken into account. In the UK these are available on prescription so that elderly and unwaged people will not pay for them and others will pay only a standard prescription charge, which at the moment represents only about 20% of the retail price of the strips. In the USA, NPT in the home is currently reimbursed by Medicare for patients with mechanical heart valves, while private insurers may cover for other indications. This article refers to Medicare, a United States insurance program. ...


There is some evidence to suggest that NPT may be less accurate for certain patients, for example those who have the lupus anticoagulant[citation needed]. Lupus anticoagulant is an autoimmune disorder caused by antibodies that bind to phospholipids and proteins associated with the cell membrane. ...


History

The prothrombin time was discovered by Dr Armand Quick and colleagues in 1935,[5] and a second method was published by Dr Paul Owren[6] (also called the "p and p" or "prothrombin and proconvertin" method). It aided in the identification of the anticoagulants dicumarol and warfarin[7], and was used subsequently as a measure of activity for warfarin when used therapeutically. An anticoagulant is a substance that prevents coagulation; that is, it stops blood from clotting. ... Dicumarol (or Dicoumarol) is a anticoagulant that functions by as a Vitamin K antagonist (similar to warfarin). ... Warfarin (also known under the brand names of Coumadin®, Jantoven®, Marevan®, and Waran®) is an anticoagulant medication that is administered orally or, very rarely, by injection. ...


The INR was introduced in the early 1980s when it turned out that there was a large degree of variation between the various prothrombin time assays, a discrepancy mainly due to problems with the purity of the thromboplastin (tissue factor) concentrate.[8] The INR became widely accepted worldwide, especially after endorsement by the World Health Organisation[9] For other meanings of the acronym WHO, see WHO (disambiguation) WHO flag Headquartered in Geneva, Switzerland, the World Health Organization (WHO) is an agency of the United Nations, acting as a coordinating authority on international public health. ...


References

  1. ^ Della Valle P, Crippa L, Garlando AM, Pattarini E, Safa O, Vigano D'Angelo S, D'Angelo A. Interference of lupus anticoagulants in prothrombin time assays: implications for selection of adequate methods to optimize the management of thrombosis in the antiphospholipid-antibody syndrome. Haematologica 1999;84:1065-74. PMID 10586206.
  2. ^ Horsti J, Uppa H, Vilpo JA. Poor agreement among prothrombin time international normalized ratio methods: comparison of seven commercial reagents. Clin Chem 2005;51:553-60. PMID 15665046.
  3. ^ a b Jackson CM, Esnouf MP. Has the time arrived to replace the quick prothrombin time test for monitoring oral anticoagulant therapy? Clin Chem 2005;51:483-5. PMID 15738512.
  4. ^ Poller L, Keown M, Chauhan N, Van Den Besselaar AM, Tripodi A, Shiach C, Jespersen J; ECCA Steering Group Members. European Concerted Action on Anticoagulation. Correction of displayed international normalized ratio on two point-of-care test whole-blood prothrombin time monitors (CoaguChek Mini and TAS PT-NC) by independent international sensitivity index calibration. Br J Haematol 2003;122:944-9. PMID 12956765.
  5. ^ Quick AJ, Stanley-Brown M, Bancroft FW. A study of the coagulation defect in hemophilia and in jaundice. Am J Med Sc 1935;190:501.
  6. ^ Owren PA, Aas K. The control of dicumarol therapy and the quantitative determination of prothrombin and proconvertin. Scand J Clin Lab Invest 1951;3:201-8. PMID 14900966.
  7. ^ Campbell HA, Smith WK, Roberts WL, Link KP. Studies on the hemorrhagic sweet clover disease. II. The bioassay of hemorrhagic concentrates by following the prothrombin level in the plasma of rabbit blood. J Biol Chem 1941;138:1-20.
  8. ^ Hirsh J, Bates SM. Clinical trials that have influenced the treatment of venous thromboembolism: a historical perspective. Ann Intern Med 2001;134:409-17. PMID 11242501.
  9. ^ Expert Committee on Biological Standardization. Requirements for thromboplastins and plasma used to control oral anticoagulant therapy. World Health Organ Tech Rep Ser 1983;33:81-105.

  Results from FactBites:
 
Prothrombin Time - [Medical Test] (258 words)
Prothrombin, or factor II, is one of several clotting factors produced by the liver.
Prothrombin time is an important coagulation test because it measures the presence and activity of five different blood clotting factors (factors I, II, V, VII, and X).
An abnormal prothrombin time is often caused by liver disease or injury or by treatment with the medication warfarin (Coumadin), which is used to prevent the formation of blood clots.
prothrombin time: Definition and Much More from Answers.com (2070 words)
The prothrombin time (PT) and its derived measures of prothrombin ratio (PR) and international normalized ratio (INR) are measures of the extrinsic pathway of coagulation.
The prothrombin time can be prolonged as a result of deficiencies in vitamin K, which can be caused by warfarin, malabsorption or lack of intestinal colonization by bacteria (such as in newborns).
The prothrombin time in hemophilia and in obstructive jaundice.
  More results at FactBites »

 
 

COMMENTARY     


Share your thoughts, questions and commentary here
Your name
Your comments

Want to know more?
Search encyclopedia, statistics and forums:

 


Press Releases |  Feeds | Contact
The Wikipedia article included on this page is licensed under the GFDL.
Images may be subject to relevant owners' copyright.
All other elements are (c) copyright NationMaster.com 2003-5. All Rights Reserved.
Usage implies agreement with terms, 1022, m