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Losing a little blood is okay. If your a normal healthy person, and got into an accident, how do they know if you need a transfusion or if you'll be okay? This is hypothetical.

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Typically transfusions are indicated for hemoglobin < 7 g/dL (for adults normally: 12-15 grams per deciliter [1]) or < 8-10 g/dL if there is cardiac disease where diminished oxygen carrying capacity cannot be compensated by increased cardiac output.

The answer above is theoretical because there is no accurate way to measure blood loss in the clinical setting - it's usually a mess.

The most useful parameter to assess intravascular volume in the immediate term is blood pressure. Again, the actual number is variable but a 30% drop from baseline in the acute setting is bad.

Another less acute indicator of hypovolemia is urine output (normal is .5-1 ml/kg body weight/hr, dropping below ~35 ml/hr therefore is an indicator of hypovolemia.)[2] Hypovolemia is often corrected with sterile saline fluid and adding packed red blood cells only as needed to keep hemoglobin in the above required range, unless someone is actively bleeding (trauma, bleeding ulcer, etc.) in which case "whole blood" (or packed red blood cells, fresh-frozen plasma and platelets) is given.[3]

[1] McPherson: Henry's Clinical Diagnosis and Management by Laboratory Methods, 22nd ed.
[2] Oliguria & Volume Depletion
[3] Fluid Resuscitation and Volume Assessment

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    $\begingroup$ Welcome to biology.stackexchange. Can you please expand your answers with some references and also explain the parameters you name? It is then easier to understand for people who are not specialized in the field. $\endgroup$ – Chris Sep 29 '14 at 22:10
  • $\begingroup$ I'm confused about the last sentence - whole blood? I took a look at the paper cited and don't see that, nor have I heard of it. Blood isn't generally stored that way. Perhaps you mean packed RBC + platelets + FFP....but that's different from "whole blood." $\endgroup$ – Susan Sep 30 '14 at 0:48
  • $\begingroup$ @Susan - One can order units of whole blood, but yes, FFP + packed red cells + platelets is more readily available and easier to transfuse. see clinicaltrials.gov/show/NCT01227005 $\endgroup$ – anongoodnurse Sep 30 '14 at 1:11
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What percent of your blood can you lose before you need a transfusion

According to Wikipedia: starting with Class III (more than 30 % of circulating volume loss), hemorrhage requires transfusion.

  • Class I Hemorrhage involves up to 15% of blood volume. There is typically no change in vital signs and fluid resuscitation is not usually necessary.
  • Class II Hemorrhage involves 15-30% of total blood volume. [...] Volume resuscitation with crystalloids (Saline solution or Lactated Ringer's solution) is all that is typically required. Blood transfusion is not typically required.
  • Class III Hemorrhage involves loss of 30-40% of circulating blood volume. [...] Fluid resuscitation with crystalloid and blood transfusion are usually necessary.
  • Class IV Hemorrhage involves loss of >40% of circulating blood volume. The limit of the body's compensation is reached and aggressive resuscitation is required to prevent death.

Of course this applies to a healthy person. If suffering from anemia or other blood conditions, smaller amounts of bleeding may require transfusion.

how do they know if you need a transfusion or if you'll be okay?

In most situations it is hard to estimate how much blood was lost. So in medical practice some indicators are used. These include (according to Puget Sound Blood Center):

  • hemoglobin / hematocrit ratio and/or values lesser than 7 / 21
  • small hemoglobin correlated with clinical signs (symptomatic anemia)
  • low O$_2$ saturation
  • acute blood loss of more than 30 % (when it can be observed)
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  • $\begingroup$ But how can the doctor tell when the patient has lost that much blood? $\endgroup$ – anongoodnurse Sep 29 '14 at 20:24
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    $\begingroup$ @anongoodnurse a significant drop in blood pressure, along with a commensurate increase in heartrate, is often used as a "back of the envelope" measure of blood loss. $\endgroup$ – MattDMo Sep 29 '14 at 23:28
  • $\begingroup$ @MattDMo - My point is that the OP asks, in trauma, "how do they know if you need a transfusion or if you'll be okay?" This answer gives no indication of how the trauma doctor would assess this. $\endgroup$ – anongoodnurse Sep 30 '14 at 0:26
  • $\begingroup$ @anongoodnurse I agree. $\endgroup$ – MattDMo Sep 30 '14 at 0:34

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