Coagulation Testing Flashcards
(40 cards)
1
Q
What does PT measure ?
A
- measures the extrinsic clotting system and the common pathway
2
Q
What does PTT measure ?
A
- measures the intrinsic pathway and common pathway
3
Q
What does thrombin time measure ?
A
- TT measures conversion of fibrinogen to fibrin polymer.
4
Q
What is a requirement of
coagulation testing ?
A
- must have citrated platelet poor plasma
- this will help prevent coagulation from being initiated because you remove the phospholipid and the calcium
read p. 71 principles of performance
5
Q
Many coagulation assays are clot-detection based,
what are some of the down sides of using an optical detection method ?
A
- optical instruments suffer from more interference by lipemia, icterus and hemolysis
6
Q
What factor deficiencies is PT sensitive to ?
A
- decreased levels of factors VII, X, V, II and fibrinogen
- Frequently used to monitor Wafarin therapy
- Started by adding Thromboplastin
- Tissue factor plus phospholipid
7
Q
What is the time frame that PT/INR testing
may be performed ?
A
- may be done up to 24 hours from the time of collection
- sample must be uncentrifuged or centrifuged with plasma remaining on top of the cells
- must be stored at 2-4 deg C or at room temp
- if testing can’t be done in 24 hours
- plasma must be removed from cells and frozen at -70 deg C
8
Q
What factors is the PT most and least
sensitive to ?
A
- Most sensitive: Factor VII
- Least sensitive: Factor II (prothrombin) and and fibrinogen
Note: most instruments have a Heparin neutralizing substance for the PT (such as polybene) in order to decrease the effect of Heparin. Also want minimal sensitivity to lupus anticoagulants.
9
Q
When there is a strong lupus
anticoagulant that interferes with the PT in a patient on
oral anticoagulation, what test is appropriate to monitor response to the oral anticoagulant ?
A
- Chromogenic factor X assay
- should be used to monitor the response to the oral anticoagulation assay.
10
Q
What is the ISI value in terms of determination
of the INR ?
A
- it is an experimentally determined value indicating the sensitivity of the thromboplastin to Warfarin induced factor deficiencies.
- Vitamin K dependent factors
- this value may be specific for instrument and reagent combinations
- use the geometric mean when the population is skewed
11
Q
What factor levels would significantly
prolong the PT ?
A
- Factors VII, V, or X : < 40%
- Fibrinogen : < 100 mg/dL
- Prothrombin : < 30%
12
Q
What is the starter for aPTT and what
does the test monitor ?
A
- partial thromboplastin
- cephalin or phosphatide
- measure the intrinsic and common coagulation pathways
- screen/monitor inherited or acquired coagulation disorders
- monitor unfractionated heparin therapy
13
Q
What factor deficiencies is aPTT less sensitive to
and what can it detect ?
A
- less sensitive to VII and XIII (cannot detect deficiencies of these)
- aPTT usually normal if all coagulation factors have a concentration of 30%
- aPTT is prolonged in:
- Heparin therapy
- Direct thrombin inhibitors
- Specific factor inhibitors
- Lupus-like anticoagulants
14
Q
How are aPTT values in newborns different
from adult values ?
A
- newborns have a prolonged aPTT
15
Q
What can cause a shortened aPTT?
A
- pre-analytical activation of clotting factors (difficult venipuncture)
- hypercoagulability
- cancer, MI
- hyperthyroidism
- diabetes
- pregnancy
- acute phase reactants
16
Q
What are some pre-analytical variables that
can affect aPTT and sometimes PT ?
A
- prolonged aPTT
- blood drawn from catheter that was flushed with Heparin
- underfilled collection tube
- prolonged PT and aPTT
- polycythemic samples - excess citrate to plasma ratio
17
Q
What does thrombin time measure ?
A
- assesses fibrinogen activity in plasma
- measures the time for clot formation when thromin is added to citrated plasma
18
Q
What can prolong the aPTT ?
A
- Heparin therapy or sample contamination (Hep lock, IV line)
- Lupus anticoagulant
- Hereditary or acquired deficiency
- Severe liver disease
- Factor or assay inhibitors
- Contact system deficiency (not associated with bleeding), Factor 12, HMWK, Pre-Kallikrein
19
Q
What can prolong the thrombin time (TT)?
A
- test is sensitive to heparin and fibrinogen
- Prolonged in:
- heparin therapy or contamination
- severe fibrin or fibrinogen degradation (DIC)
- dysfibrinogenemia
- thrombin inhibitor:
- lepirudin
- drugs
- antibodies
20
Q
How does the
Platelet function analyzer (PFA) work?
A
- whole blood is drawn through a drug coated opening
- Collagen/Epinephrine
- Collagen/ADP
- measure the time to occlusion of the orifice
21
Q
What can increase the PFA
closure times?
A
- Thrombocytopenia
- Anemia
- Antiplatelet drugs
- von Willebrand disease
- other hereditary or acquired platlet dysfunction
22
Q
What is the mechanism of the antithrombin
activity assay and what is it measuring ?
A
- measures antithrombin activity
- Chromogenic substrate assay (anti-IIa assay)
- plasma gets diluted in a heparin buffer and excess thrombin (IIa) is added
- residual thrombin is measured with a chromogenic substrate
- difference between original and final is equal AT activity
23
Q
What conditions can decrease
antithrombin activity ?
A
- Heparin therapy
- ongoing or extensive thrombosis
- systemic coagulation activation (DIC)
- nephrotic syndrome
- Hereditary deficiency
- Aspariginase therapy
24
Q
What is the methodology for measuring
Protein C activity and what can
decrease Protein C activity ?
A
- Chromogenic assay
- Protein C activity decreased in :
- Warfarin therapy, Vitamin K deficiency
- Liver disease
- Ongoing extensive thrombosis
- DIC
- Hereditary deficiency
25
How is protein S measured ?
* Screen with Free Protein S antigen
* Quantitative immunoassay
* Protein S activity
note:
* there is interference from elevated levels of Factor VIII (falsely low S), and thrombin inhibitors like Lepirudin (falsely high S)
26
What situations can decrease the protein S levels
as well as the protein S activity ?
* Warfarin therapy
* Vitamin K deficiency
* Liver disease
* ongoing or extensive thrombosis
* systemic coagulation (DIC)
* pregnancy
* Inflammatory syndromes due to increase in C4B biniding protein
* Hereditary deficiency
27
How is the Lupus inhibitor assay
performed ?
* perform a lupus sensitive PTT with and without addition of excess hexagonal phase phospholipid
* calculate the difference in clotting time with and without hexagonal phospholipid
28
How is the activated protein C
resistance assay performed ?
* PT based assay
* using Factor V activation and inhibition by APC
* Calculate the ratio with and without APC added
* Reduced ratio indicates APC resistance
* Factor V Leiden
29
How is the vWF antigen assay performed ?
* quantitative automated latex immunoassay
30
How is vWF factor function evaluated ?
* enzyme immunoassay
* antibody is directed to the functional site on vWF
* Highly correlated with Ristocetin cofactor assay
* Type 1: reduction in parallel with vWF antigen
* Type 2: greater reduction than vWF antigen
31
How is the vWF collagen binding assay performed ?
* collagen coated plate, plasma vWF binds to collagen
* bound vWF detected with anti-vWF antibody
* Type 1: reduction in parallel with vWF antigen
* Type 2: greater reduction than vWF antigen
32
How are the vWF multimers evaluated ?
* uses a western blot assay of the von willebrand factors
* Type 2A: loss of intermediate and high MW multimers, abnormal forms
* Type 2B: loss of high MW multimers, normal pattern
33
What lab values should make you think that the
sample has been heparinized ?
* PTT and TT are prolonged
* this is also the pattern seen with a direct thrombin inhibitor
34
What can cause a prolonged PT and normal PTT ?
* deficiency or inhibitor of Factor 7
* early Warfarin therapy (Factor 7 falls first)
* Atypical Lupus inhibitor
35
What can cause a prolonged PTT and a normal PT ?
* Lupus inhibitor
* deficiency or inhibitor of Factor 8, 9, 11, or 12 or contact phase
36
What can cause prolongation of both PT and PTT ?
* Warfarin therapy, Vitamin K deficiency, liver disease
* Deficiency or inhibitor to: Factor 2, 5, or 10
* Thrombin inhibitor
* Fibrinogen deficiency
37
What causes a prolonged Thrombin time ?
* Hypofibinogenemia (usually \<90)
* Dysfibrinogenemia
* The presence of fibrin or fibrin split products that block the conversion of fibrinogen to fibrin
* unfractionated Heparin
* LMW Heparin usually does not cause prolonged TT
* Presence of thrombin inhibitors
* Presence of high immunoglobulins which can prevent fibrin formation
Note: if PTT is prolonged, can obtain a TT to rule out or rule in Heparin contamination
38
What are the uses for Fibrinogen assays ?
* diagnosis of dysfibrinogenemia and hypofibrinogenemia
* diagnosis of DIC, liver failure and primary fibrinolysis
* guiding transfusion therapy with cryoprecipitate or NovoSeven
39
What is the definition of a Bethesda unit ?
* quantity of inhibitor that neutralizes 50% of the Factor VIII in normal plasma in 2 hours
* quantification of the Bethesda assay --\> serial dilutions to help identify an inhibitor level to a factor
* low titer: 0.5 to 5 BU
* high titer: \> 5 BU
40
What are the specimen transport and handling conditions
often required for coagulation testing?
* transported at temps 15-22 deg C
* too cold of temperatures can activate platelets and the clotting cascade
* too hot of temperatures can degrade the clotting factors
* For plasma based testing:
* plasma should be separated within 4 hours of sample draw
* For Heparin activity testing
* 1 hour requirement for plasma separation
* Plasma should be separated for most coagulation tests
* platelets can activate coagulation
* must double centrifuge for Lupus anticoagulant testing
* Repeat freeze-thaw events may result in loss of Factor VIII and Factor V testing
