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.
Typically transfusions are indicated for hemoglobin < 7 g/dL (for adults normally: 12-15 grams per deciliter ) 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.) 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.
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)