Platelet clumping occurs when the blood platelets responsible for coagulation stick to one another to form clusters. The presence of platelet clumping has no clinical consequences other than preventing instruments from properly counting blood platelets.
Platelet clumps tend to form in samples collected with difficulty and in a small number of individuals (about 1/2000) whose platelets react with the EDTA anticoagulant in lavender-top tubes. When the number of platelets measured by the instrument is normal, the presence of platelet clumping is of little clinical significance. When the number of platelets is below normal, a microscope platelet count is automatically performed to ensure that the low number of platelets was not due to the presence of a large number of clumps. In such a situation, the number of platelets has to be interpreted using a manual count.
When interference with the EDTA in lavender-top tubes is suspected, it is recommended that the analysis be repeated with a tube containing citrate, another anticoagulant found in blue-top tubes that significantly reduces such interference.