Calculating Allowable Blood Loss
Most published methods for calculating allowable blood loss (ABL) assume that the hemoglobin concentration of the last drop of blood lost is the same as the hemoglobin concentration of the first drop of blood lost. During surgery, blood loss is continuously replaced with crystalloids such as 0.9% saline or lacated Ringer's solution (at a ratio of 4 cc crystalloid for every 1 cc blood lost) or colloid such as human albumen or starch solutions (at a ratio of 1 cc colloid for every 1 cc blood lost) in order to maintain circulating volume with a net drop in hematocrit during the surgery.
Although we could just measure the hemoglobin periodically, it is often inconvenient to do so, let alone costly if performed multiple times. It is helpful, therefore, to have an accurate method of estimating the Allowable Blood Loss before and during a surgery. Bourke and Smith, in 1974, published an article in Anesthesiology entitled 'Estimating allowable hemodilution', in which they showed a more accurate set of formulae which took into account ongoing hemodilution during the surgery as follows: