The invention relates to a system for holding a fluid sample, said system comprising a transparent flexible tube for holding said fluid sample, a tube holder for holding said tube, a first flattening element, and second flattening element, wherein said first flattening element and said second flattening element may be moved relative to each other thereby changing said transparent flexible tube from a first state to a second state, where at least a first cross sectional dimension of said tube is smaller in said second state than in said first state.