- Thrombocytopenia documented by a platelet nadir less than 150, 000 cells/mm³. Thrombocytopenia is subclassified as class one HELLP syndrome: platelet nadir ≤50, 000 cells/mm³, class two HELLP syndrome: platelet nadir ≤100, 000 cells/mm³, or class three HELLP syndrome: platelet nadir ≤150, 000 cells/mm³..