Main Laboratory (2M): Monday-Friday, 8:00 am-5:00 pm
Emergency (STAT) Service: 24 hours/7 days
Hematology – Information and Results: 206-8590
TEST CONSULTATION AND APPROVAL
Some coagulation tests are performed ONLY after consultation with a Laboratory Medicine Resident. See the Alphabetical List of Tests for further information. A Laboratory Medicine Resident is available at all times for coagulation and bleeding problems:
Monday – Friday, 8 am to 5 pm; pager number: (415) 443-0179.
At all other times, page the Laboratory Medicine Resident on-call at pager number (415) 443-6969, or call the Clinical Laboratory Information Section, 206-8590, to contact the Resident on-call.
All Coagulation specimens should be submitted on a Main Laboratory, Blood/Serum (2M) Requisition.
BLOOD COLLECTION TUBES USED
Color of Cap
Whole Blood Drawn
Contents and Use Capability
0.3 mL of 3.2% (109 mM) buffered sodium citrate, used for all coagulation tests requiring citrated plasma.
0.2 mL of 3.2% (109 mM) buffered sodium citrate; use when smaller amount of blood is drawn.
COMMENTS ON COLLECTING COAGULATION SPECIMENS
DO NOT DRAW from heparinized intravenous or arterial lines. Heparin affects all coagulation results.
Tissue thromboplastin released during the venipuncture is a powerful activator of the clotting system and should not be allowed to contaminate any coagulation specimen. It is therefore important to do a clean venipuncture.
When multiple blood tubes are collected, the order of collection is important. Tubes for coagulation studies (blue top) should be drawn before serum tubes (red top), because the clot activator in plastic serum tubes may cause interference in coagulation testing. Glass tubes that do not contain any additives (also red top) may be drawn before the coagulation tube.
One full tube is sufficient for DIC work-up, i.e., Prothrombin Time (PT), Activated Partial Thromboplastin Time (APTT), Fibrinogen, and Fibrin D-Dimer (FDD). One tube is also sufficient for one or two factor assays. For inhibitor screens or complete factor work-up, draw two full tubes.
Buffered sodium citrate chelates calcium to anticoagulate the blood and maintains the proper plasma pH. Centrifugation takes away platelet phospholipids from the plasma. Thus, each clotting test has to add back standard amounts of calcium and phospholipids to resume the clotting process in a controlled test environment.
The ratio of blood to anticoagulant is ideally maintained at 9:1, or 2.7 mL blood to 0.3 mL sodium citrate in a light blue top tube (1.8 mL blood to 0.2 mL sodium citrate in the smaller light blue top tube). This ratio is based on a normal hematocrit of 45%. A short sample (less than 2.7 mL or, for the smaller light blue top tube, less than 1.8 mL) or high hematocrit (greater than 55%) will result in an excess of anticoagulant, thereby prolonging results.
A correction formula and nomogram have been developed that allow adjustment of the citrate anticoagulant volume to compensate for higher than normal hematocrit values (CLSI Document H21-A5, Wayne, PA: Clinical Laboratory Standards Institute, 2008).
The Laboratory stocks specially prepared collection tubes in which the citrate volume has been reduced to achieve more accurate coagulation testing in patients with hematocrit values from 56 to 70%. These special blue top tubes are available at the Nursery (6H). Each tube contains 0.17 mL of sodium citrate and should be filled only up to the black line (2.25 mL of whole blood).
NOTE: THIS TUBE HAS NO VACUUM, so blood should be collected with a syringe then transferred into the tube up to the black line.
An adjustment has also been suggested when the patient's hematocrit is below 20%. However, there is not enough data to document a significant difference if the adjustment is not made. Of prime consideration here is the increased risk of the specimen clotting, since there is less anticoagulant in relation to the patient's increased plasma volume.
Under-filling blue top tubes can prolong both PT and APTT. If the tube is filled less than 2.7 mL (or less than 1.8 mL in the case of the smaller blue top tubes) a long PT or APTT can become significantly longer than the true value. A normal PT or APTT result, however, may be only slightly longer than the true value.
As the impact of under-filling on coagulation studies is difficult to predict, under-filled tubes will not be accepted for testing.
Send the specimen to the Clinical Laboratory immediately. Ideally, specimens for coagulation should be received and processed within one hour of collection. This is especially important for assessment of anticoagulation with unfractionated heparin. See Prothrombin Time (PT), Partial Prothrombin Time, activated (APTT), Fibrinogen, and Factor Assays for test-specific information on specimen stability.