vi) Mix by tapping the plate gently and then allow the RBCs to settle for about 40 minutes at
room temperature, i.e. about 20°C, or for 60 minutes at 4°C, if ambient temperatures are
high, by which time control RBCs should have formed a distinct button.
vii) HA is determined by tilting the plate and observing the presence or absence of tearshaped
streaming of the RBCs. The titration should be read to the highest dilution giving
complete HA (no streaming); this represents 1 HA unit (HAU) and can be calculated
accurately from the initial range of dilutions.