Serum; a serous membrane.
[Latin serum, whey.]
Serum is the amber-coloured, protein-rich liquid which separates out when blood coagulates, for which the associated adjective is serous; serology is its scientific study or diagnostic examination. A seropositive individual has a positive result in a test of blood serum, say for the presence of a virus, while someone seronegative tests negative; seroconversion is the change from the latter state to the former. The seroprevalence of a pathogen is its level in a population, as measured in blood serum. Serotonin is a neurotransmitter present in blood platelets and serum.