Patient preparation instructions: Specimens should be collected no earlier than 15 minutes from onset or up to 3 hours after the onset of the suspected incident causing mast cell activation. The time between the reaction and sample collection should be noted. To confirm the return to baseline levels an additional blood sample should be collected after 24-48 hours, time depending on the magnitude of the activation. At the suspicion of elevated basal levels or an underlying mastocytosis additional sample(s) should be taken 1-2 weeks(s) later.