HEMOSTASIS AND COAGULATION

• A 65-year-old woman came to the ED with swollen legs and shortness of breath
• A chest x-ray and venogram where ordered
• The physician made a diagnosis of deep vein thrombosis and pulmonary embolism
• The woman was hospitalized and started on heparin
• The woman was also given Coumadin at 10 mg/day
• On days 3 and 5 after starting Coumadin, her blood was drawn for PT/INR
• The specimen arrived in the laboratory, it was centrifuged, and it was processed by an automated analyzer using photo-optical end-point detection methodology
• Prothrombin Time (PT)


  A test that measures the time it takes for a clot to form
• International Normalized Ratio (INR)


  A standardized way of reporting PT results
• The following results were obtained:

  Test

  Results - Day 3

  16.7 sec/2.6

  Results - Day 5

  7.3 sec/0.4 (Flag)

  Reference

  Interval

  10.9-13.0 sec

  time/INR

  Prothrombin
• The laboratory's policy is to investigate all flagged results and to retest all unusual coagulation results
• The day 5 prothrombin time assay was repeated and similar values were obtained
• The clinical coagulation laboratory is an ever-changing environment populated by automated analyzers that offer advances in both volume and variety of tests
• For many years, the routine coagulation test menu consisted of only prothrombin time (PT) with the international normalized ratio (INR), partial thromboplastin time (PTT; also referred to as the activated partial thromboplastin time [APTT]), fibrinogen, and thrombin time assays
• More specialized testing was performed in tertiary care institutions or reference laboratories employing medical laboratory scientists with specialized training
• With the introduction of new instrumentation and test methodologies, coagulation testing capabilities have expanded significantly, so many formerly "specialized" tests now can be performed easily by general medical laboratory staff
• New instrumentation has also made coagulation and expanded hemostasis testing more standardized, consistent, and cost-effective
• Automation has not advanced, however, to the point of making coagulation testing foolproof or an exact science
• Operators must develop expertise in correlating critical test results with the patient's diagnosis and when monitoring antithrombotic therapy
• Good method validation of procedures, cognitive ability, and theoretical understanding of the hemostatic mechanisms are still required to ensure the accuracy and validity of test results so that the physician can make an informed decision about patient care
• Visual clot-based testing began in the 18th century
• The first observation of blood clotting was from blood taken from the vein of a dog; the sample was completely "jellied" in about 7 minutes
• In 1780 Hewson measured that human blood clotted in 7 minutes, using a basin to collect the blood
• With the discovery of the microscope, scientists were able to observe visible clot formation and turbidity
• Many advances in the assessment of blood clotting took place from 1822 to 1921
• In the early 1900s researchers monitored the length of time it took whole blood to clot in a glass tube while it was being tilted, a precursor to the Lee-White clotting time (1913)
• These early clotting time tests depended on observation of the clot directly (visually) or microscopically
• In 1910 the first clot detection instrument, the "Koaguloviskosimeter," was developed by Kottman
• Hartert developed the thromboelastograph (TEG) in 1948, and a related clot detection device based on sonar clot detection followed
• Citrated plasma (usually platelet-poor plasma, or plasma with a platelet count of less than 10,000/µL) came to replace whole blood in coagulation testing
• Except for a few specific coagulation tests (e.g., point-of-care testing, for which whole blood is still used), citrated plasma remains the specimen of choice for the largest volume of coagulation testing
• Plasma coagulation testing began in 1920 when Gram added calcium chloride to anticoagulated plasma at 37° C
• He measured the increasing viscosity of the plasma during fibrin monomer polymerization, laying the groundwork for the PT and PTT assays
• In these early days and for many years hence coagulation testing, largely via only the PT and PTT assays, was typically performed by adding plasma and reagents to a glass tube held in a 37° C water bath
• Clot formation was determined by visual inspection of the plasma as the tube was tilted, and a stopwatch was used to determine the time to clot formation
• This was referred to as the tilt-tube technique
• The first instruments dedicated to coagulation testing were nephelometers, developed in 1920
• These devices measure 90-degree light dispersion of a colloidal suspension
• As plasma clots, a change in light scatter can be measured over time, a principle still in use today
• The 1950s witnessed the dawn of the modern era of instrumentation in coagulation testing with the development of the first coagulometer, the BBL Fibrometer