Understanding blood test

Blood tests alone can't be used to diagnose many diseases or medical problems. But blood tests can help you and your doctor learn more about your health. Blood tests also can help find potential problems early, when treatments or lifestyle changes may work best.

Blood tests show whether the levels of different substances in your blood fall within a normal range.

Many factors can cause your blood test levels to fall outside the normal range. Abnormal levels may be a sign of a disorder or disease. Other factors—such as diet, menstrual cycle, how much physical activity you do, how much alcohol you drink, and the medicines you take (both prescription and over-the-counter)—also can cause abnormal levels. Your doctor should discuss any unusual or abnormal blood test results with you. These results may or may not suggest a health problem.

Parameter Reference range Significance
Red blood cells (varies with altitude)   
In addition to nutritional deficits, may be decreased in those with hemorrhage, hemolysis, genetic aberrations, marrow failure, or renal disease, also who are taking certain drugs. Not sensitive for iron, vitamin B12, or folate deficiencies. 
A high RBC count may occur because red blood cell production increases to compensate for low oxygen levels due to poor heart or lung function, compensate for lower oxygen levels at higher altitudes, the kidneys release too much of a protein, the bone marrow is producing too many red blood cells, the loss of blood plasma, often due to sodium and water depletion.
Hemoglobin (varies with altitude)
Men: 14-17 g/dl 
Women: 12-15 g/dl 
Pregnant: <11 g/dl
Blood loss and bone marrow suppression reduce total RBC count and therefore lower total hemoglobin content. Other patients have a normal RBC count but a low hemoglobin level. This situation occurs with iron-deficiency anemia. Women need more iron in their diets than men, due to the regular loss of iron in the menstrual flow.
High hemoglobin count may occur because red blood cell production increases to compensate for chronically low blood oxygen levels due to poor heart or lung function, you smoke, red blood cell production increases to compensate for a limited oxygen supply in higher altitudes, bone marrow dysfunction.
Hematocrit (varies with altitude)
Men: 41-51% 
Women: 36-44% 
Pregnant: <33%
May be decreased in those with: burns, dehydration, shock, cardiovascular disease, chronic lung disease, congenital heart defects, cushing's disease, hepatic (liver) cancer, polycythemia vera.
Low values could be a result of: anemia, overhydration, addison's disease, bone marrow suppression, chronic infection, hemodilution, hodgkin's disease, leukemia, multiple myeloma, rheumatic fever, subacute bacterial endocartitis, systemic lupus erythematosus, vitamin deficiency (B6, B12, folic acid).

Mean cell volume (MCV)   
80-99 fl

Decreased in presence of iron deficiency, thalassemia, trait and chronic renal failure, anemia of chronic disease. 
Increased in presence of vitamin B12 or folate deficiency and genetic defects in DNA synthesis.
Mean cell hemoglobin (MCH)
27-31 pg/cell
The MCH is increased in and decreased in the same conditions as the MCV.
Mean cell hemoglobin concentration (MCHC)
32-36 g/dl
Decreased in those with iron deficiency and thalassemia trait. Not sensitive to marginal nutrient deficiencies.

White blood cell count (WBC)
5-10 x 103/mm3 
(>2 yr)
6-17 x 103/mm3 
(<2 yr)
Increased in those with infection, neoplasia.
Decreased in those with autoimmune diseases, overwhelming infections, who are receiving chemotherapy or radiation therapy.
Random distribution of width (RDW)
This value tells how consistent are the size of the red blood cells. Newly made cells (reticulocytes), B12 and folic acid deficient cells are larger than iron deficient cells. This is an electronic index that may help clarify if an anemia has multiple components. The high RDW helps determine if there is only a B12 and/or folic acid deficiency (with normal RDW showing the red cells are mostly the same size) or with concomitant iron deficiency (a high RDW due to small and large red blood cells).
The RDW is decreased in those with iron deficiency anemia, vitamin B6 anemia, rheumatoid arthritis.
The RDW is increased in those with vitamin B12 and pernicious anemia, folic acid anemia, iron deficiency anemia combined with other anemia, hemolytic anemia, transfusions, sideroblastic anemia, alcohol abuse.
55-70% neutrophils 
20-40% lymphocytes 
2-8% monocytes 
1-4% eosinophils 
0.5-1% basophils
Neutrophils: ketoacidosis, trauma, stress, puss-forming infections, leukemia. 
: PEM, aplastic anemia, chemotherapy, overwhelming infection. 
: infection, leukemia, myeloma, mononucleosis. 
: leukemia, chemotherapy, sepsis, AIDS. 
: tuberculosis, subacute bacterial endocarditis, syphilis, rickettsial,recovery from neutropenia, leukemias, myeloproliferative disorders, lymphomas, multiple myeloma, collagen vascular diseases, chronic ulcerative colitis, sprue, myositis, polyarteritis, temporal arteritis.
: acute infections, stress, treatment with glucocorticoids, aplastic anemia, hairy cell leukemia, acute myeloid leukemia, treatment with myelotoxic drugs and genetic syndromes. 
: parasitic infestation, allergy, eczema, leukemia, autoimmune disease. 
: increased steroid production.
: leukemia. 
: allergy.
Platelet count (PLT)
140-450 x 103/mm3
Platelets are fragments of cells that participate in clotting. People whose platelet count is low bleed easily after brushing their teeth, small cuts, surgeries. Platelets are decreased in those with marrow depression (aplastic anemia, radiation, chemotherapy, drugs), marrow infiltration (acute leukemia, carcinoma, myelofibrosis, multiple myeloma), megaloblastic anemia (B12 and/or folic acid deficiency), increased destruction (drugs (chemotherapy, heparin , dilution due to overhydration, coagulation disorders, hypersplenism, rubella, liver dysfunction (cirrhosis), idiopathic cytopenic purpura (ITP).
Platelets are increased in those with infection, acute blood loss, disseminated carcinoma, splenectomy, various free radical pathologies (tissue damage, chronic inflammation, surgery), thrombocythemia, polycythemia vera, myeloproliferative disorders, chronic granulocytic leukemia, hemolytic anemia.


  1. Anonymous9/13/2013

    Is there anything I can do to improve results of my blood test?

    1. Maintain your weight, and focus on getting in as much healthy, nutritious food as possible. Give your body all of the vitamins and minerals it needs from a wide variety of healthy foods.

  2. Anonymous9/13/2013

    If I have an abnormal result on my blood test, what other tests might my doctor order as follow up?

    1. It depends what is wrong with your blood test. A blood test (complete blood count, CBC) is used to detect many different health conditions. It may be used to diagnose infections or allergies, blood clotting problems or blood disorders (anemia), evaluate red blood cell production or destruction.

  3. Anonymous9/13/2013

    Does a normal blood test indicate no bacterial infection?

    1. When a person has a bacterial infection, the number of white cells rises very quickly. If in your CBC white cells are in normal range, it seems you don't have bacterial infetion.

  4. Anonymous9/13/2013

    Can taking a calcium supplement with breakfast and dinner decrease iron absorption?

    1. Calcium makes it harder for the body to absorb iron either from food or supplements. However, in people who have enough stored iron, this probably isn't a problem. But if you are iron deficient or might become iron deficient, minimize this interaction by separating your intake of calcium and iron. Don't take calcium supplements at mealtime or when you take iron supplements.

  5. Anonymous9/13/2013

    What is a complete blood count?

    1. A complete blood count (CBC) is a blood test used to evaluate your overall health and detect a wide range of disorders, including anemia, infection and leukemia.

  6. Anonymous9/13/2013

    What is the complete blood count used for?

    1. The complete blood count is often used as a broad screening test to determine an individual's general health status.

  7. Anonymous9/13/2013

    How is blood collected for a CBC?

    1. A blood sample is obtained by inserting a needle into a vein in the arm or from a fingerstick (for children and adults) or heelstick (for infants).