Anemia means loss of blood (red blood cells). This can occur from an obvious injury, or internal bleeding, usually in the gastrointestinal (G-I) tract, the long “tube” including stomach and bowel (see Diagram — Gastrointestinal System). G-I bleeding may not look red; vomited blood may look like coffee grounds, and blood mixed with stool (poop) may look pitch black (iron tablets and medicines like Pepto Bismol® also make stools black).
Some rare diseases can make red blood cells break apart (“hemolysis”), causing anemia, and also jaundice (yellow eyes and skin). Anemia only causes symptoms if it’s severe, or occurs suddenly. Headache is a common symptom; another is new shortness of breath while walking. Diagnosis is easy, by a simple blood test for Hemoglobin (usually ordered as a Complete Blood Count (CBC).
The only reason for a false-negative is if the bleeding began within the last few hours. That’s because the body needs to maintain a certain level of volume inside our arteries so that blood can circulate efficiently. To do so, it moves fluids from areas around organ cells into the bloodstream, which takes time.
I had a young woman come to my E.R. with new shortness of breath. Her blood pressure was bottoming-out, in Shock, but her hemoglobin level was normal! We found her anemia by giving her 1000 ml. of IV fluids (for shock) and repeating the test. Her positive pregnancy test diagnosed the cause — a severe ruptured Ectopic Pregnancy, where the bleeding went up into the belly instead of out the vagina (she hadn’t even noticed a missed menstrual period, which isn’t uncommon).
In cases like the above, there’d be some abnormality in vital signs to alert us, like a rapid heart rate. If that isn’t present, we have the patient lie flat for 5 minutes, then stand abruptly. If blood volume is inadequate, the heart speeds up within 1-2 minutes. See postural vital signs).
Suspecting and finding anemia is one thing. But it can be harder to identify the cause. See Anemia.