A blood sample typically clots when the natural coagulation process begins prematurely or due to improper collection and handling. This renders the sample unsuitable for most laboratory tests, as the results would be unreliable.
Common Causes of Blood Sample Clotting
Blood samples require careful handling to prevent clotting, especially when collected for tests that require whole blood or plasma. Based on standard laboratory practices, several factors can contribute to a blood sample clotting, including:
- Blood Slow to Fill the Tube: If the blood flow into the collection tube is too slow, the blood has more time to activate its natural clotting cascade before it adequately mixes with the anticoagulant present in the tube.
- Prolonged Use of a Tourniquet: Leaving a tourniquet on the patient's arm for an extended period (typically more than one minute) can lead to hemoconcentration and local activation of the coagulation system, making the blood more prone to clotting once drawn.
- Samples Incompletely Mixed: Most blood collection tubes contain an anticoagulant (like EDTA or sodium citrate) designed to prevent clotting. After blood collection, these tubes must be gently inverted a specific number of times immediately. If the sample is not adequately mixed, the anticoagulant will not disperse throughout the blood, allowing portions of the sample to clot.
- Syringe Collection and Slow Transfer of Sample: When blood is collected using a syringe and then transferred to collection tubes, any delay in transferring the blood can lead to clotting. Blood's exposure to the air and the surface of the syringe can activate clotting factors before it reaches the anticoagulant in the tube.
- Not Enough Blood in the Tube: Blood collection tubes are designed to hold a specific volume of blood to ensure the correct blood-to-anticoagulant ratio. If there is insufficient blood in the tube, the anticoagulant may be too concentrated, or the blood might not mix effectively with the anticoagulant, leading to clotting or inaccurate test results.
Consequences of Clotted Samples
When a blood sample clots, particularly for tests intended for hematology (e.g., Complete Blood Count), the sample is unusable. Haematology laboratories routinely reject clotted samples because the presence of clots indicates that the blood components are no longer uniformly distributed. This makes all test results derived from such a sample unreliable, potentially leading to misdiagnosis or incorrect treatment plans.
