Detailed analysis of red blood cells (RBCs) aggregation is often required in various clinical studies. Most conventional aggregation indices are dimensionless values and not available for comparison of across studies. Quite recently, we have developed microfluidic aggregometry that enables us to yield a critical shear-stress that are required to aggregate RBCs under the shearing hydrodynamic force. The present study investigated the relationships between the values of the critical shear-stress and conventional aggregation indices by comparing the critical shear-stress measured by the microfluidic aggregometry with the threshold shear-stress measured using a LORCA aggregometer. The results showed that the critical shear-stress did not vary with the hematocrit value while the threshold shear-rate decreased with the hematocrit value. The threshold shear-stress also showed the same hematocrit-independence as the critical shear-stress. These findings assist in rheologically validating the critical shear-stress, as defined in the microfluidic aggregometry, within the present range of hematocrit values.