Lab Interpretation: Low hemoglobin, hematocrit in adults

ALGORITHM — 

(algorithm 1)

IMMEDIATE ACTION — 

Rapidly identify and triage individuals with potentially life-threatening anemia, including those with:

●Hemodynamic instability

●Hemorrhage

●Brisk or new-onset hemolysis

●End-organ ischemia

●Emergencies associated with pancytopenia (table 1)

If any of these factors are present, obtain:

●Type and crossmatch

●Complete blood count (CBC) with differential, platelet count

●Reticulocyte count

●Prothrombin time (PT), activated partial thromboplastin time (aPTT)

●Liver biochemical tests (including total and indirect bilirubin, alanine aminotransferase [ALT], aspartate aminotransferase [AST])

●Blood urea nitrogen (BUN), creatinine

Examination of the peripheral smear by an experienced individual may identify abnormalities not detected by automated machines.

Life-saving interventions, including hydration and hemodynamic support, should not be delayed while awaiting the results of diagnostic testing.

Hematologic consultation is appropriate to:

●Assess new-onset hemolysis, cytopenias, and worrisome abnormalities of cell lines identified by the CBC or peripheral blood smear (picture 1 and picture 2). (See "Evaluation of the peripheral blood smear".)

●Guide urgent therapy. (See "Pretransfusion testing for red blood cell transfusion", section on 'Emergency release blood for life-threatening anemia or bleeding'.)

INITIAL EVALUATION — 

Patients not requiring immediate action require a timely evaluation to determine whether the patient has occult bleeding, evidence for increased red blood cell (RBC) destruction (hemolysis), bone marrow suppression, or iron or other nutrient deficiency (table 2). The patient's history, prior CBCs (if available), symptoms, and initial laboratory results guide additional testing to determine the cause. Most cases of anemia can be diagnosed by classifying the anemia according to RBC size (mean corpuscular volume [MCV]) and the reticulocyte count. (See "Diagnosis of hemolytic anemia in adults", section on 'High reticulocyte count'.)

Obtain (review):

●Repeat CBC with differential and platelet count to determine the rate of fall of the hemoglobin and hematocrit

●Absolute or corrected reticulocyte count

In patients with declining hemoglobin/hematocrit and a normal or increased corrected reticulocyte count, assess for blood loss or hemolysis. If occult bleeding is the source of anemia, the MCV is usually normal or low, whereas hemolysis typically is associated with a normal or high MCV. (See 'Normal mean corpuscular volume (80 to 100 fL)' below and 'Low mean corpuscular volume <80 fL' below.)

Request hematologic consultation to evaluate anemia associated with absent or very low reticulocyte count, new-onset hemolysis, or worrisome abnormalities of other cell lines.

Examination of the peripheral blood smear by a trained eye may identify abnormalities not detected by automated machines but is not required in all patients (eg, not required in iron deficiency anemia diagnosed by iron studies). A blood smear examination may identify:

●A small population of RBCs with distinctive size or shape abnormalities in early microcytic or macrocytic anemia (when the MCV is still normal)

●Schistocytes, spherocytes, or sickle forms due to acute or chronic hemolysis

Low mean corpuscular volume <80 fL — Distinguish between the two most common causes of microcytic anemia: iron deficiency and, in certain populations, thalassemia (table 2). (See "Diagnosis of thalassemia (adults and children)", section on 'Rule out iron deficiency' and "Anemia of chronic disease/anemia of inflammation", section on 'Iron studies' and "Diagnostic approach to anemia in adults", section on 'Evaluation based on MCV'.)

Obtain (review):

●Serum iron

●Total iron binding capacity (TIBC)

●Ferritin

Determine source of bleeding if iron deficiency is present (table 3). If unexplained, assess for occult gastrointestinal bleeding. Correct iron deficiency prior to assessing for suspected thalassemia.

Normal mean corpuscular volume (80 to 100 fL) — Assess for common causes of normocytic anemia, which include bleeding and anemia of chronic disease (table 2). (See "Anemia of chronic disease/anemia of inflammation", section on 'Diagnostic evaluation' and "Diagnosis of hemolytic anemia in adults", section on 'Diagnostic approach' and "Diagnostic approach to anemia in adults", section on 'Evaluation based on MCV'.)

Obtain (review):

●Iron, TIBC, ferritin

●Liver biochemical tests (including ALT, AST, total and indirect bilirubin)

●Chemistries (including BUN, creatinine)

●C-reactive protein and/or erythrocyte sedimentation rate

In patients with suspected hemolysis, obtain:

●Total and indirect bilirubin

●Lactate dehydrogenase (LDH)

●Haptoglobin

●Direct antiglobulin (Coombs) test

In patients with suspected bleeding of unknown etiology, assess for occult gastrointestinal bleeding.

High mean corpuscular volume >100 fL — Assess for common causes of macrocytic anemia, including chronic alcohol use, hemolysis, and nutrient deficiency. (See "Clinical manifestations and diagnosis of vitamin B12 and folate deficiency", section on 'Diagnostic evaluation' and "Diagnosis of hemolytic anemia in adults", section on 'Diagnostic approach' and "Nutritional status in patients with sustained heavy alcohol use".)

Obtain (review):

●Vitamin B12, folate

●Liver biochemical tests (including ALT, AST, total and indirect bilirubin)

●Thyroid-stimulating hormone (TSH)

For patients with B12 or folate deficiency, the peripheral blood smear may show a megaloblastic blood picture (picture 3). (See "Low vitamin B12 and low folate in adults".)

In patients with a normal or increased reticulocyte count, assess for hemolysis as well as bleeding.

Obtain:

●Total and indirect bilirubin

●LDH

●Haptoglobin

●Direct antiglobulin (Coombs) test

REFERENCE RANGE — 

In adults, the normal reference ranges for RBC parameters vary by gender, race, and age (table 4). The lower limits of normal for the hemoglobin level range from 13.0 to 14.2 g/dL (130 to 142 g/L) for men and 11.6 to 12.3 g/dL (116 to 123 g/L) for women. The normal ranges may not apply to certain populations (eg, athletes, people living at high altitudes, smokers). Interpretation of a specific abnormal test result should be based upon the reference range reported by the laboratory. (See "Diagnostic approach to anemia in adults", section on 'Caveats for normal ranges'.)

CITATIONS — 

The supporting references for this content are accessible in the linked topics.