A problem to be solved by the present invention is to provide an immunochromatographic test strip and a detection method using immunochromatography avoiding aggregation of colloidal gold conjugates while red blood cells in whole blood are agglutinated and then separated and removed in the case of using polybrene as a blood-agglutinating agent and the colloidal gold conjugates as a detection reagent. To solve the problem, the present inventers reviewed a past reagent configuration itself from a completely different viewpoint rather than selecting type and amount of polyanions and, as a result of extensive study on each element, the inventers surprisingly found that aggregation of colloidal gold can be suppressed by using a certain buffer solution without using neutralization by polyanions.