- Specimen should be sent to the laboratory immediately, to ensure testing within 4 hours of collection. These must NOT be transported through the Lamson tube.
- Testing will be performed on samples up to 5 hours old but the results might be compromised
- One full 2.7 ml Citrate sample or 2 x 1.8 ml is required for the PFA.
- Sample must be a venous collect, micro collects are not suitable.
- If there is a request for additional haemostasis tests, check the test guide for specimen requirements.
The test is performed in two stages:
- The epinephrine/collagen test is performed first. If this is normal, no further testing is necessary.
-
If the result is prolonged, we will proceed to the ADP/collagen test.
Closure time with Epinephrine/Collagen cartridge: 75 - 190 seconds.
Closure time with
ADP/Collagen
cartridge: 55 - 130 seconds.
Uncertainty of measurement : 15%
This assay gives an overall measure of platelet function.
Principle
Blood is drawn through a capillary opening at high sheer stress which mimics the conditions in the normal circulation. The capillary is coated with one of two agonists; either collagen and epinephrine or collagen and ADP. The agonist activates the platelets, which aggregate and occlude the capillary. The analyser measures the time to occlusion, which is reported as the "closure time".
When should I ask for this test?
The test is primarily used in the investigation of patients with a significant history of bleeding. It is particularly sensitive to the concentration of von Willebrand Factor and is therefore a good screening test in suspected cases of von Willebrands disorder. It also has a role in the investigation of rare platelets disorders such as Glanzmanns disease and Bernard-Soulier syndrome.
Do antiplatelet drugs affect the result?
In general aspirin causes significant prolongation of the closure time, but results can be quite variable. There are several studies underway in the field of cardiology to see if the PFA can be used to monitor aspirin therapy to ensure that angina patients are on adequate treatment. The PFA is affected by any drugs that alter platelet function including non-steroidal anti-inflammatories, ticlopidine, persantin, and GPIIb/IIIa inhibitors.
Does anything else affect the result?
Patients with thrombocytopenia have a long closure time and there is some evidence the results are prolonged in the presence of a low haematocrit. The closure time is shorter in newborns and shorter in pregnancy. The PFA is not affected by heparin.
Requesting a PFA
Please provide clinical information, particularly personal and family history of bleeding and details of present medication. Specifically ask about aspirin as it is in many proprietary cough and cold medicines. Aspirin can affect platelet function for up to 10 days.
For further information contact the Haematology laboratory (Ext 22067) or:
| Dr Nicola Eaddy | Ext 22005 | |
| Dr Peter Bradbeer | Ext 22062 | |
| Dr Anna Ruskova | Ext 22137 | |
| Dr Nikhil Rabade | Ext 22071 |