haematocrit has 2 distinct synonyms spread across 2 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the ratio of the volume occupied by packed red blood cells to the volume of the whole blood as measured by a hematocrit
a measuring instrument to determine (usually by centrifugation) the relative amounts of corpuscles and plasma in the blood
Synonyms, antonyms and the broader and narrower relations for haematocrit come from Princeton University’s WordNet. Full sources and licences.