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Thread: [Laboratory] Blood Coagulation Tests

  1. #1
    PharmD Year 1 TomHsiung's Avatar
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    Default [Laboratory] Blood Coagulation Tests

    D-Dimer
    Normal range: <0.5 mcg/mL or <200 ng/mL but varies with specific assay

    D-dimer is a neoantigen formed when thrombin initiates the transition of fibrinogen to fibrin and activates factor XIII to cross link the fibrin formed. This neoantigen is formed as a result of plasmin digestion of cross-linked fibrin. The D-dimer test is specific for FDPs (fibrin degradation products), whereas the formation of fibrinolysis may be either fibrinogen or fibrin derived following plasmin digestion.

    The D-dimer is often used to diagnose or rule out thrombosis in the initial assessment of a patient suspected of having acute thromboembolism; results are typically elevated if a patient is positive for VTE. However, D-dimer is a sensitive, but nonspecific, marker for VTE because other causes such as malignancy, DIC, infection, inflammation, and pregnancy can also elevate the D-dimer levels; thus, a positive result does not necessarily confirm a diagnosis of VTE, but a negative result can help rule out a VTE.

    In addition to diagnose or rule out VTE, D-dimer has been used for its predictive value for recurrent thromboembolism in patients treated for first event idiopathic VTE. Studies have shown that patients with normal levels of D-dimer after treatment for first event idiopathic VTE have a low risk for VTE recurrence, whereas abnormal levels of D-dimer are predictive of VTE recurrence. Thus, in patients with abnormal levels of D-dimer, an extended duration of anticoagulation therapy could be considered.

    D-dimer is also a common test used to diagnose and evaluate patients with DIC.
    B.S. Pharm, West China School of Pharmacy, Class of 2007, Health System Pharmacist, RPh. Hematology, Infectious Disease.

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    PharmD Year 1 TomHsiung's Avatar
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    Default Re: [Laboratory] Blood Coagulation Tests

    Methods of Correcting the Reticulocyte Count for the Degree of Anemia


    • Reticulocyte count = % reticulocytes in red blood cell (RBC) population
    • Corrected reticulocyte count = % reticulocytes x (patient Hct/45)
    • Reticulocyte production index = Corrected reticulocyte count / maturation time in peripheral blood in days*


    *Reticulocyte maturation time = 1 day for Hct >=40%; 1.5 days for Hct 30-40%; 2.0 days for Hct 20-30%; 2.5 days for Hct <20%
    B.S. Pharm, West China School of Pharmacy, Class of 2007, Health System Pharmacist, RPh. Hematology, Infectious Disease.

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    PharmD Year 1 TomHsiung's Avatar
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    Default Iron Related Tests

    Serum Ferritin
    Normal range: >10-20 ng/mL or >10-20 mcg/L
    Loss of storage iron (hemosiderin) was traditionally evaluated by iron-stained bone marrow aspirate. Serum ferritin has largely replaced these invasive tests as an indirect measure of iron stores. Serum ferritin concentrations are markedly reduced in iron deficiency anemia (3-6 mcg/L).

    Serum Iron and Total Iron-Binding Capacity
    Normal range: 60-150 mcg/dL or 10.7-26.9 umol/L
    Total iron-binding capacity (TIBC) normal range: 250-400 mcg/dL or 45-72 umol/L

    The serum iron concentration measures iron bound to transferrin. This value represents about one third of the total iron-binding capacity (TIBC) of transferrin. The TIBC measures the iron-binding capacity of transferrin protein.

    In iron deficiency anemia, TIBC is increased due to a compensatory increase in transferrin synthesis. This increase leads to a corresponding decrease in the percent transferrin saturation that can be calculated by dividing the serum iron by the TIBC, and then multiplying by 100. Iron deficient erythropoiesis exists whenever the percent saturation is 15% or less.

    Other disease states besides iron deficiency that can alter serum iron and TIBC are infections, malignant tumors, and uremia. Serum iron and TIBC both decrease in these disorders, unlike in iron deficiency anemia where serum iron decreases but TIBC increases. Anemia from these diseases is sometimes called anemia of chronic disease.
    B.S. Pharm, West China School of Pharmacy, Class of 2007, Health System Pharmacist, RPh. Hematology, Infectious Disease.

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    PharmD Year 1 TomHsiung's Avatar
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    Default Re: [Laboratory] Blood Coagulation Tests

    What is INR? What are the factors affecting PT?

    Look at this equation,

    [Laboratory] Blood Coagulation Tests-screen-shot-2017-03-11-at-9-58-38-pm-png

    The PT is dependent on the thromboplastin source and test method used to detect clotting. Thromboplastin reagents are derived from animal or human sources and include recombinant products. Factor sensitivity is highly dependent on the source of the thromboplastin, and can exhibit variability between different lots of the same reagent. Some thromboplastin reagents are less sensitive to changes in factor activity. This means that it takes a more significant decrease in factor activity to produce a prolongation of the PT. Differences in reagent sensitivity, combined with the influence of endpoint detection, affect clotting time results both in the normal and therapeutic ranges. Large differences in factor sensitivity between comparative methods can result in conflicting interpretation of results, both in the assessment of factor deficiencies and adequacy of anticoagulation therapy.


    • Thromboplastin Source
    • Citrate Concentration


    Since PT results can vary widely depending on the thromboplastin source, a standardized reporting method has been used, which is known as the international normalized ratio (INR). The INR is calculated according to the above equation. The international sensitivity index (ISI) expresses the sensitivity of the thromboplastin reagent compared to the World Health Organization (WHO) reference standard. The more sensitive or responsive the reagent, the lower the ISI; reagents with ISI values of <1.7 are recommended for use when monitoring patients on oral anticoagulant therapy. Theoretically, an INR result from one laboratory should be comparable to an INR result from a different laboratory, even though the Pos may be different. The citrate concentration also may affect the ISI determination of certain reagents, with higher citrate concentrations leading to higher INR results; using blood samples anti coagulated with 3.2% citrate, instead of higher concentrations, can help mitigate this problem.


    • Heparin
    • Others


    There are other factors that may influence the PT. If heparin-sensitive thromboplastin reagents are used, falsely elevated PT/INR values may result. These inaccurate values might suggest sufficient anticoagulation with oral anticoagulation therapy and result in the premature discontinuation of heparin.

    Although the INR system has greatly improved the standardization of the PT, one can still expect differences in INRs reported with two different methods, particularly in the upper therapeutic and supra therapeutic ranges. The greater the differences in the ISI values for two comparative methods, the more likely differences will be noted in the INR. Laboratories and anticoagulation clinics should review the performance characteristics of the PT method used to evaluate their specific patient populations and report changes in methods to healthcare professionals, particularly those monitoring anticoagulant therapy.

    Heparin may also prolong PT since it affects factor II (FII/FIIa) in the common pathway; the addition of a heparin neutralizing agent to the blood sample can blunt this effect at heparin concentrations up to 2 units/mL. However, at higher concentrations of heparin, whether due to higher doses of heparin or sample collection issues, the neutralizing agent may not be enough and the PT may be prolonged. These "crossover" effects may have to be considered when oral and parenteral anticoagulants are given concomitantly for several days to avoid premature discontinuation of the parenteral agent.
    Last edited by TomHsiung; Sat 11th March '17 at 10:41pm.
    B.S. Pharm, West China School of Pharmacy, Class of 2007, Health System Pharmacist, RPh. Hematology, Infectious Disease.

  5. #5
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    Default Re: [Laboratory] Blood Coagulation Tests

    Prothrombin Time and the INR

    Definition
    • The PT assesses the coagulation activity of the extrinsic and common coagulation pathways.
    • Tissue thromboplastin (tissue factor) is used as a potent activator of the coagulation system in the presence of added calcium. Currently, recombinant tissue factor is used in most commercial reagents. The potency of tissue factor explains the shortness of clotting (in seconds) in the assay.
    • Normal ranges:
      • PT: 9.6-12.4 seconds (may vary slightly from laboratory to laboratory)
      • INR: 1.0 ratio (remains constant independent of equipment or reagent used)

    Use
    • Evaluation of clotting disorders that may involve the extrinsic coagulation mechanism (factor VII) and the common pathway (factors II, V, X, and fibrinogen). In these situations, the PTT should be ordered in parallel with the PT. PT is not sensitive to clotting factors if they are modestly decreased (>30%). In addition, it is not sensitive to abnormalities in factors involved in the intrinsic coagulation pathway (factors XII, XI, IX, and VIII) or in protein C or S deficiencies.
    • Evaluation of liver function reflecting abnormalities in factors VII, II, X, IX and V (but not VIII).
    • To monitor long-term oral anticoagulant therapy with coumarin and indanedione derivatives. PT is prolonged >PTT, and more consistently so. Factor V is not affected by anticoagulants, whereas it may be decreased in liver disease.

    Interpretation
    • Marked prolongation of the PT in liver disease indicates advanced disease.
    • Marked elevation of INR in patients receiving vitamin K antagonists is a marker of excessive anticoagulation and requires prompt action contrariwise, and INR below 2.0 reflects insufficient anticoagulation.
    • Combined abnormal PT and PTT is found under two circumstances:
      • Medical: administration of oral anticoagulants, DIC, liver disease, vitamin K deficiency, massive transfusions
      • Coagulation factor abnormalities: dysfibrinogenemias; factors V, X, and II defects
    B.S. Pharm, West China School of Pharmacy, Class of 2007, Health System Pharmacist, RPh. Hematology, Infectious Disease. Chengdu, Sichuan, China.

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