This testing tool includes at least a flow channel through which a sample containing red blood cells and a substance to be measured is circulated. The flow channel includes at least a sample supply part to which the sample is supplied, a
blood cell separation membrane for separating the red blood cells from the sample, and a detection part for detecting the substance to be measured in the sample. The detection part is located downstream of the
blood cell separation membrane in a flow direction of the sample, and at least one
polymer of a poly(
meth)
acrylic acid having a weight average molecular weight of 35,000 to 1,000,000 inclusive and a poly(
meth)
acrylic acid inorganic salt having a weight average molecular weight of 35,000 to 1,000,000 inclusive is contained in at least a part of the region
lying between the sample supply part and the
blood cell separation membrane in the flow direction of the sample in the flow channel.