Blood synthehol level referred to the concentration of synthehol present in one's blood.
In 2375, despite her protests that it was impossible, The Doctor measured Seven of Nine's blood synthehol level and found it to be 0.05 percent, diagnosing her with intoxication. Upon hearing that she'd only drunk one glass of champagne, he surmised that the Borg couldn't hold their liquor and offered her inaprovaline for her symptoms. (VOY: "Timeless")