About Blood Calculator:

This "Blood Calculator" is meant to serve as a guide in estimating blood loss in post-operative patients, especially children. A 200 cc blood loss may be inconsequential in an adult, but may be life-threatening in an infant. But knowing how much blood loss is too much after surgery can be difficult to estimate. That is one way in which this calculator can help. The other way it can help is estimating roughly how much blood is needed for a transfusion in order to reach a desired hematocrit.

Disclaimer 1: This calculator is meant to serve only as a guide and not as a true clinical decision-making tool. The author assumes no responsibility for its use or misuse, and the user of the calculator must assume all responsibility for patient care.

Disclaimer 2: The calculator bases its results on many assumptions which likely do not hold up in real-life situations. Thus, the true answer in any patient may deviate significantly from what this Blood Calculator provides. Some of the assumptions are explained below

Part 1: Blood loss estimation

The first part of the program helps to answer the following two general questions:

1. Suppose a 35 kg patient's pre-operative hematocrit was 38, and his post-operative hct is 28. The surgeons reported an estimated blood loss (EBL) of 200 cc. Does that amount of blood loss explain the drop in hematocrit, or might something else being going on with the patient that needs to be explored?

2. Suppose a 15 kg child had a pre-operative hematocrit of 35 and the estimated blood loss was 200 cc. When the post-op hematocrit is obtained later, what would we expect it should be based on that amount of blood loss?

It is important to note what assumptions are made and what has not been taken into account:

1. The patient is considered to be a closed system steady state. This means that it does not take intravascular fluid changes into account such as inputs (intravenous fluid, blood transfusions in the operating room, etc.) or outputs (urine output, respiratory losses, third-spacing/edema, etc.). It also means that once blood is lost, the volume of fluid that the patient lost was replaced with fluids before a post-operative hematocrit is obtained. It also means that whatever blood was lost in the operating room, it had the same hematocrit as the pre-operative value.

2. Other relevant parameters such as blood pressure are not taken into account. Thus, a patient's hematocrit could be unchanged in the event of a sudden, acute blood loss which might only first be evident by a drop in blood pressure or urine output.

Use of the program should be self-explanatory, but it may help to demonstrate it's use through some examples:

Example 1

A 70 kg adult male has a pre-operative hematocrit of 40 and 78 cc/kg of blood in the intravascular volume. His total intravascular blood volume is then 70 x 78 = 5460 cc. The total volume of blood cells would then be 40% of this volume (since the hematocrit is 40), which results in 5460 * 40% = 2184 cc of blood cells (without the plasma). The patient returns from the operating room with a reported blood loss of 600 cc.

Knowing this amount of blood loss, what we expect the post-operative hematocrit to be?

This estimated blood loss (EBL) was 600 cc's of the patient's original blood which had a hematocrit of 40. Therefore, the patient lost 600 * 40% = 240 cc of blood cells. He now has 2184 - 240 = 1944 cc of blood cells remaining. Since we assume that the blood volume has achieved its normal steady state value of 5460 cc, the new hematocrit can be estimated to be 1944 / 5460 = 0.36 = 36%. Thus, we assume only a slight drop in hematocrit.

Example 2

A 12 kg, 24 month-old girl has just returned from the operating room. Her pre-operative hematrocrit was 35 and a hematocrit done in the recovery room was 30. What would we expect the reported estimated blood loss (EBL) to be?

We assume that her initial blood volume was 78 cc/kg, or 12 x 78 = 936 cc. Since her hematocrit was 35, her total volume of red blood cells would be 936 * 35% = 328 cc. If her post-operative hematocrit was 30, she would have ended up with 936 * 30% = 281 cc of red blood cells left. That means she lost approximately 328 - 281 = 47 cc of red blood cells. Since it is assumed that the blood loss was based on her original hematocrit, she would have lost about 47 / 35% = 135 cc of blood.

Part 2: Blood transfusion estimation

This part of the program is simple to use. If there is a post-operative hematocrit entered from part 1, it will automatically be placed in the "current hematocrit" box, although this can easily be changed. You must then enter the desired hematocrit and the hematocrit of the packed red blood cells (PRBCs) you are using (which often is 65%). The calculations are performed as follows:

Suppose you have a 60 kg female adult with an estimated blood volume of 78 cc/kg of total blood volume. That means she has a total blood volume of 4680 cc. If her current hematocrit is 30, then she has a total of 4680 * 30% = 1404 cc of red blood cells. Assuming that the desired hematocrit is 40, it means that she should have 4680 * 40% = 1872 cc of red blood cells. Her current deficit is 1872 - 1404 = 468 cc of red blood cells. Therefore should would need 468 / 65% = 720 cc of packed red blood cells (that have a hematocrit of 65%) to provide enough red blood cells to bring her total hematocrit back to 40 (assuming that she retains equilibrium with her total intravascular volume).


Please e-mail the author with any questions, concerns, etc. Postive feedback is accepted too! If you have ideas to improve it, that would be greatly appreciated.

Created on August 17, 2001. Last revised on August 31, 2004.