He's had a series of tests at the hospital but they can't find anything wrong.

series 是名词，意思是"系列、连续"，指按顺序排列或连续发生的一组事物。在这里指"一系列医疗检查"。 series 本身是单复数同形的名词，既可以表示单数概念也可以表示复数概念。