Question 1

Which of the following is an acceptable sample for a prothrombin time (PT)test?

Accepted Answer

PT specimens may be held 18° to 24°C (room temperature)for 24 hours from the time of collection and still give valid results.Any clot in a specimen makes the sample useless for coagulation testing.Serum separator tubes contain a clot activator;this can contaminate the next tube drawn,and,for coagulation,is the same as having a clot.Heparinized samples are not used for coagulation testing.

Question 2

Which of the following is necessary for normal platelet aggregation?

Accepted Answer

Normal platelet aggregation requires fibrinogen.Ristocetin is a substance added in vitro that causes platelet to clump.VWF is needed for normal platelet adhesion.Epinephrine is a platelet agonist stimulating platelet aggregation,but it is not necessary.

Question 3

How often must the partial thromboplastin time (PTT)reference interval be determined?

Accepted Answer

The PTT reference interval must be determined with each new lot of reagent and at least once per year.

Question 4

Which of the following will cause spuriously prolonged clot-based coagulation tests?

Accepted Answer

Filling an evacuated 3.2% sodium citrate only halfway (50%)will lead to falsely prolonged clot-based coagulation test results.The relative excess anticoagulant will bind (some)of the calcium added in the test system.A 90% fill gives valid results and can be used.Prolonged tourniquet application and plasma collected from a traumatic tap both have the potential of activating coagulation,thus leading to shortened,not prolonged,results.

Question 5

Which of the following is an acceptable sample for testing on a photo-optical clot-detecting instrument?

Accepted Answer

Plasma should be collected at a 9:1 ratio of blood to anticoagulant (3.2% sodium citrate).Lipemia and hemolysis are possible interfering substances when using an instrument that uses the photo-optical principle for end-point detection.Serum cannot be used for coagulation testing because the sample has already clotted,with resultant loss of several coagulation proteins.

Question 6

Which of the following is good practice in regard to phlebotomy on patients with hemostasis testing ordered?

Accepted Answer

For routine coagulation draws,a 20- or 21-gauge needle is used.The patient does not need to be fasting,the sodium citrate tube should be drawn first,and glass (syringe)should never be used for coagulation specimens because glass activates the contact phase.

Question 7

A patient sample has normal platelet aggregation patterns for all agonists except that no reaction occurs with ristocetin.Ristocetin agglutination is not corrected with exogenous VWF.What condition is most likely?

Accepted Answer

Platelets from patients with Bernard-Soulier syndrome do not agglutinate with ristocetin,and it is not corrected when exogenous VWF is added.Their platelets have an intrinsic membrane defect,a deficiency of the GP1b or IX portion of the GP1b/V/IX receptor.This is in contrast to platelets from patients with VWD;they also do not agglutinate with ristocetin,but agglutination is corrected when exogenous VWF is added.Platelets from patients with storage pool disorder or those taking aspirin agglutinate normally with ristocetin.

Question 8

A patient admitted to the hospital with thrombosis is given the standard heparin dosage.Twenty-four hours after the heparin is started,the PTT is 38 seconds (reference range,25 to 37 seconds).Both normal and abnormal controls are within their limits.Which of the following is most likely?

Accepted Answer

When a patient has a deficiency of antithrombin,heparin may be administered in therapeutic or even higher dosages,but it will not exert an anticoagulant effect in vivo or be detected by the PTT.This is called heparin resistance.

Question 9

Which of the following conditions would have a normal PTT?

Accepted Answer

Factor VII deficiency would have a normal PTT.All the other listed conditions would have a prolonged PTT.

Question 10

A patient is stabilized on warfarin with an international normalized ratio (INR)of 2.7.He develops pneumonia and is admitted to the hospital.His appetite is depressed and he is placed on broad-spectrum antibiotic agents.At 1 week after the admission,his INR is 5.0.What is the most likely interpretation?

Accepted Answer

This elevated INR is probably the result of a deficiency of vitamin K,which increases the effects of warfarin.Vitamin K deficiency is seen during use of broad-spectrum antibiotic agents that destroy gut flora and in people with a poor diet,both of which are present in this patient.

Question 11

How often must controls be run for the PT and PTT?

Accepted Answer

Controls should be run at the beginning of each 8-hour shift and with each change of reagent for both of these tests.

Question 12

Which of the following practices is necessary when collecting blood specimen from a heparin or saline lock?

Accepted Answer

Collection of blood from a heparin or saline lock is usually performed by physicians or nursing staff.The IV line must be flushed with 5-mL saline before the blood draw to avoid specimen contamination.The first 5 mL of blood that is collected should be discarded.IV lines should never be flushed with heparin before collecting blood for coagulation testing.Blood should be collected into a syringe,and a transfer device is used to transfer the blood to an evacuated tube.

Question 13

A platelet aggregation study shows diminished response to all agonists except for a small response to arachidonic acid and full response to ristocetin.Which condition is most likely?

Accepted Answer

Glanzmann thrombasthenia is diagnosed by its diminished secretion and aggregation responses to all agonists except a modest response to arachidonic acid and full,although nondiagnostic,response to ristocetin.

Question 14

Which of the following would likely have a normal PT?

Accepted Answer

A patient with a factor VIII deficiency would have a normal PT.All the others listed would have a prolonged PT.

Question 15

The platelet function assay (PFA-100)will be normal and is no value to perform in patients who have

Accepted Answer

The platelet function assay (PFA-100)will be normal in patients with a factor VIII deficiency.It ideally will be prolonged in all the other listed settings.The bleeding time test is being used less often because it has poor predictive value and leaves scarring.

Question 16

A prolonged PTT is corrected when mixed with normal plasma and tested immediately and after a 2-hour incubation.Which test (or tests)should be performed next?

Accepted Answer

This pattern suggests a factor deficiency,and factor VIII and IX assays should be performed.The Bethesda titer is performed when a specific factor inhibitor is present;no evidence exists for this because the PTT on the incubated 1:1 mix did not prolong.High-phospholipid PTT testing is done to confirm the lupus anticoagulant;this is not present because the immediate 1:1 mix on this sample corrected.No reason exists to perform an anti-Xa assay,often used to monitor LMWH.

Question 17

Patients with which of the following would have a normal PT?

Accepted Answer

Patients with a deficiency of factor IX will have a normal PT.The PT does not require this protein.

Question 18

A patient's platelet count drops 4 days after being placed on heparin.Select the correct interpretation.

Accepted Answer

This pattern,a drop in the platelet count 4 days after being placed on heparin,suggests heparin-induced thrombocytopenia.It could also be a benign,limited thrombocytopenia related to heparin.Assays such as the immunoassay for heparin-dependent anti-PF4 can distinguish the two.

Question 19

What is the test commonly used to monitor the high heparin doses used in cardiac bypass surgery?

Accepted Answer

The high heparin doses used in cardiac bypass surgery can be monitored using the PTT.

Question 20

A patient has a hematocrit of 65%.Which of the following is true related to coagulation testing?

Accepted Answer

The plasma volume is relatively low in a patient who has a high hematocrit.This unacceptably raises the anticoagulant-to-plasma ratio,invalidating test results that become falsely prolonged.When the hematocrit is greater than 55%,the volume of citrate must be decreased to optimize the anticoagulant-to-plasma ratio.Blood is not collected in heparin for coagulation tests.

Quiz 42: Laboratory Evaluation of Hemostasis

Which of the following is an acceptable sample for a prothrombin time (PT) test?