Lab Investigation of Coagulation Disorders...including Reflex Tests Flashcards
Mixing studies
- Principle of the test
- If PT/APTT are normal, stop!
- But if either or both are prolonged and can’t be explained by disease or drugs, then mixing studies are ordered
- Determines if results are due to either factor deficiency or inhibitor to coagulation
Mixing studies
- Testing procedure
- Determine which part of the cascade is affected…possible factors
- Rule out heparin contamination
- Mix patient plasma w/ pooled normal plasma (PNP)
- Proceed w/ confirmatory testing
Mixing studies
- Interpretation of results
After ruling out heparin contamination, and mixing w/ PNP:
- Correction: suspect a factor deficiency
- No correction: suspect an inhibitor
How do you determine the presence of heparin?
Add protamine sulfate or hepzyme to the patient’s plasma and rerun abnormal test:
- If tests are normal: stop, the increase was due to heparin
- If tests are still prolonged: proceed
How do you determine the presence of a deficient factors?
Make a mixture of 50% patient: 50% PNP and run the mixtures
- If mixture immediately corrects the abnormal results to w/in 10% of normal, suspect a factor deficiency
- If mixture does not correct, suspect inhibitor (circulating anticoagulant)
How do you determine the presence of a circulating anticoagulant?
If 50:50 mixture does not correct, suspect an inhibitor (circulating anticoagulant)
- Specific factor inhibitors or
- LLAs (lupus-like anticoagulants)
Factor Assay
- Principle
The degree to which the patient plasma corrects (or fails to correct) a specific factor deficient substrate…is compared to a reference curve/standard curve
Functional Fibrinogen Assay
- Principle
(Modification of the TCT)
- When “excess” thrombin is added to “dilute” patient’s plasma, the time required for a clot to form is inversely proportional to the fibrinogen concentration (less fibrinogen would be slow to clot/more fibrinogen would be quick to clot)
Two classifications of circulating anticoagulants
- Specific factor inhibitors
- LLAs (lupus-like anticoagulants)…aka non-specific inhibitors or antiphospholipid Abs
Specific factor inhibitors
- Definition
Ab that destroys a specific factor in progressive, time-consuming fashion
Specific factor inhibitors
- Occurrence
- Secondary to factor infusions used in Hemophilia A and B treatment (approximately 30%)
- Secondary to txn
- Spontaneously
Specific factor inhibitors
- Lab results (screening and mixing studies)
Screening: - Normal PT - Prolonged APTT Mixing studies: - PNP does NOT correct (times will get progressively longer w/ extended incubation)
Specific factor inhibitors
- Confirmatory test(s)
Bethesda titer (to quantify inhibitor)
LLAs
- Definition
Antiphospholipid Abs, aka LLAs, are blood proteins that appear following a viral infection or during the course of a chronic inflammatory condition like arthritis (e.g., lupus) or cancer
LLAs
- Occurrence
- Detectable in 1-2% of people, but usually disappear w/in 6 weeks
- Those whose LLAs do not disappear present w/ a 30% risk of thrombosis
LLAs
- Lab results (screening and mixing studies)
Screening: - Prolonged PT - Prolonged APTT Mixing studies - PNP does NOT correct - Times do NOT get progressively longer w/ extended incubation
LLAs
- Confirmatory tests
- Platelet neutralization test (PNT)
- Dilute Russell’s Viper Venom Test (dRVVT)
LLAs
- In vitro effects
- In vivo effects
In vitro
- Inhibit the assembly of prothrombinase complex (Xa, Va, Ca2+, PL)
In vivo
- No effect
Method to screen for a suspected deficiency of Fletcher factor
- Rerun APTT and extend incubation to 15 minutes (instead of the usual 3-5 minutes)
- If abnormal time “partially corrects” think of PK deficiency
- WHY: allows time for “feedback mechanism” to help correct abnormal clotting time
Typical results for deficiency of Fletcher factor
If abnormal time “partially corrects” think of PK deficiency
- WHY: allows time for “feedback mechanism” to help correct abnormal clotting time
What test is used to confirm a Factor XIII deficiency?
Urea Solubility Test
Expected test results for a vascular disorder
- Platelet count
- PT
- APTT
- BT
- Platelet count: Normal
- PT: Normal
- APTT: Normal
- BT: Abnormal
Expected test results for a fibrinolytic disorder (acquired):
- Platelet count
- PT
- APTT
- BT
- Platelet count: Abnormal
- PT: Abnormal
- APTT: Abnormal
- BT: Abnormal
Expected test results for a quantitative platelet disorder:
- Platelet Count
- PT
- APTT
- BT
- Platelet Count: Abnormal
- PT: Normal
- APTT: Normal
- BT: Abnormal
Expected test results for a qualitative platelet disorder :
- Platelet Count
- PT
- APTT
- BT
- Platelet Count: Normal
- PT: Normal
- APTT: Normal
- BT: Abnormal
Expected test results for a Factor disorder:
- Platelet Count
- PT
- APTT
- BT
- Platelet Count: Normal
- PT: Abnormal (dependent on factor def)
- APTT: Abnormal (dependent on factor def)
- BT: Normal
What are some possible causes of the following test results:
- APTT: abnormal
- PT: abnormal
- Thrombin time: normal
- Vitamin K deficient
- Liver disease
- Inhibitor present
- Factor deficiency in common pathway (X, V, III)
What are some possible causes of the following test results:
- APTT: abnormal
- PT: normal
- Factor deficiency in intrinsic pathway (XIII, Xi, IX, VIII, Fletcher, Fitzgerald)
- Lupus anticoagulant
- Specific factor inhibitor
What are some possible causes of the following test results:
- APTT: normal
- PT: abnormal
- Factor deficiency in the extrinsic pathway (VII)
- Specific factor inhibitor
What are some possible causes of the following test results:
- APTT: abnormal
- PT: abnormal
- Thrombin time: abnormal
- Factor I deficiency
- Severe liver disease
- DIC
- Potent inhibitor
- Hypofibrinogenemia or dysfibrinogenemia
What are some possible causes of the following test results:
- APTT: normal
- PT: normal
- Thrombin time: normal
- Factor XII deficiency
- Specific factor inhibitor
- Normal patient or lab error
