Dangerously low hemoglobin levels that require transfusion are 7 grams per deciliter for ICU patients and 8 milligrams per deciliter for most other patients, according to U.S. News. Low hemoglobin is anything below 13.5 grams per deciliter for men, and 12 grams per deciliter for women, according to Mayo Clinic.
Patients who have hemoglobin levels of 9 to 10 grams per deciliter and receive transfusions do not appear to recover better than people who receive transfusions at 7 to 8 grams per deciliter. Hemoglobin levels that are just below normal levels normally aren't a cause for concern, according to Mayo Clinic. Sometimes low hemoglobin levels are caused by disease or a serious medical condition, which is referred to as anemia.