Whole blood cell count – CBC

by Kaja Cankar and Gregor Prosen



The whole blood cell count is one of the most commonly ordered tests in medicine. It is a routine hematological screening study, performed to evaluate the status and overall health of a patient.

Whole blood cell count includes total red blood cell count (RBC) with indices, hemoglobin (Hb), hematocrit (HCT), white blood cell count (WBC) with or without a differential and a platelet level.


Simplified guidelines suggest that whole blood cell count is indicated for

  • trauma patients with acute blood loss and GCS of 8 or below,
  • patients with acute illnesses of non-traumatic origin,
    • which includes patients in emergency room presenting with fever, chest pain, abdominal pain, gastrointestinal bleeding, constipation, severe diarrhea, vomiting, severe nosebleed, irritability and crying (infants), throat pain, hypertensive urgencies and emergencies, severe joint pain or low back pain, skin rash, scrotal pain, seizures, syncope, vaginal bleeding, weakness and patients with lightning injuries, patients after near-drowning and patients with terrestrial venomous bites and stings.

Red blood cell (RBC) count and indices

RBC count and indices can assist in determining whether our patient has anemia, polycythemia, and erythrocytosis.

  • Hemoglobin (Hb) reflects the amount of hemoglobin or oxygen-carrying potential available in the blood.
  • Hematocrit (Hct) indicates the proportion of whole blood that is occupied with red cell mass.
  • Special measurements of red cells, called indices, include:
    • MCV Mean corpuscular volume or average size of the red cell.
    • MCH Mean corpuscular hemoglobin or average hemoglobin content.
    • MCHC Mean corpuscular hemoglobin concentration or average hemoglobin concentration.
    • RDC Red cell distribution width measures the range of cell size.

White blood cell count

WBC count includes differential with detail information about neutrophils, lymphocytes, monocytes, eosinophils, and basophils.


Platelet level is the number of platelets or thrombocytes in a given volume of whole blood. Both increased and decreased levels can point to abnormal conditions of excess clotting or bleeding.


Variations In The Test Results

In a hospital setting, it is important to avoid taking blood from the same side as an infusion in order to avoid hemodilution. It should be taken into consideration that some samples that were difficult to obtain, e.g., lengthy venipuncture using a narrow gauge needle, such as a small butterfly, may result in abnormalities due to cell lysis or clotting.

CBC in trauma patients may sometimes be misleading. It presents normal initial levels of hemoglobin which do not exclude a significant hemorrhage. Patient’s hemoglobin value is not a real-time indicator of his or hers intravascular blood volume, and it takes quite some time (minutes-hours) before hemoglobin value reflects the degree of blood loss in trauma patients accurately. Following the trend of serial hemoglobin measurements, every 15 to 30 minutes can provide useful information regarding ongoing blood loss.

In trauma patients elevated white blood cell can often be found, but this occurrence is unlikely due to infection. WBC is elevated due to demargination of WBCs during the stress response.


Interpretation of Test

  • Increased WBC
    • Infection (localized and generalized)
    • Inflammation (i.e., vasculitis)
    • Myeloproliferative disorder
    • Tissue necrosis (burns)
    • Myocardial infarction
    • Physiological stress (e.g., exercise, pain, surgery, prolonged crying in infants)
    • Medications (steroids)
    • Vomiting
    • Dysrhythmias
    • Acute myocardial infarction (AMI),
    • Pregnancy

The physician should look for a “left shift” which indicates the presence of immature forms in the peripheral circulation (bands). Usually, this represents an infectious state.

  • Decreased WBC
    • Infection (overwhelming sepsis or viral),
    • Underlying hematopoietic disease (aplastic anemia, agranulocytosis)
    • Immunosuppression,
    • Medications (antibiotics, chemotherapeutic agents)

Patient presenting with neutropenia is at risk of infections from common and opportunistic organisms.

  • Decreased HCT
    • Blood loss
    • Hemolysis
    • Long-standing anemia
    • Pregnancy

If suspecting acute loss, the physician should look for schistocytes on the peripheral blood smear. Long-standing anemia can be evaluated by the RBC indices. Administration of fluids in hypovolemic patients or trauma resuscitation will cause a decreased HCT.

  • Increased HCT
    • Hemoconcentrated states (dehydration, burns, diarrhea)
    • High altitude,
    • Exercise,
    • Polycythemia Vera
    • Chronic obstructive lung disease
  • Decreased Hb
    • Iron deficiency, vitamin deficiencies, e.g., vitamin B12
    • Bleeding,
    • Kidney disease
    • Inflammatory disorders (rheumatoid arthritis or infections)
    • Hemolysis (accelerated loss of red blood cells through destruction)
    • Inherited hemoglobin defects (thalassemia or sickle cell anemia)
    • Cirrhosis of the liver
    • Bone marrow failure and cancers that affect the bone marrow
  • Causes of increased Hb are similar to HCT.
  • Increased platelet count
    • Myeloproliferative diseases
    • Malignancy
    • Infection
    • Recent surgery (splenectomy)
    • Chronic inflammation (i.e., irritable bowel syndrome)
    • Trauma (massive hemorrhage, thrombus)
    • Secondary to iron deficiency anemia or hemolytic anemia

In thrombocytosis, there is an excess of platelets (more than 1 million), but they are usually large and nonfunctioning.

  • Decreased platelet count
    • Infections (SBE, HIV, septicemia, mononucleosis)
    • Drug-induced destruction (penicillin, heparin, sulfonamide, quinine)
    • Idiopathic, thrombocytopenic purpura, thrombotic thrombocytopenic purpura, disseminated intravascular coagulation
    • SLE
    • Toxemia of pregnancy
    • Renal insufficiency
    • Bone marrow failure due to carcinoma, leukemia, lymphoma, or fibrosis
    • Other: menses, poor nutritional states such as iron, folate, and vitamin B12 deficiencies.


Hints And Pitfalls

  • Patients with serious infections may have completely normal, or even low WBC counts. Overreliance on normal WBC counts in the setting of acute infections may lead to misdiagnosis and delays in patient care.
  • Toxic granulations, Döhle Bodies, and cytoplasmic vacuolization are remnants of phagocytosis found in neutrophils. These are indicative of more serious bacterial infections.
  • Cutoff values of white blood cell (WBC) counts greater than 15,000/mm3 suggest a higher likelihood of serious illness.
  • Acute hemorrhage will not be reflected in the Hgb or HCT early on.
  • Stress can cause the level of white blood cells to elevate, which can be misinterpreted as an infection.
  • In patients presenting with abdominal pain, an elevated WBC does not necessarily imply a serious disease.

Pediatric, Geriatric, Pregnant Patient, And Other Considerations

  • Geriatric patients will more than likely demonstrate normal to low WBC counts in sepsis.
  • Elevated WBC can be found in prolonged crying in infants, pain, vomiting dysrhythmias, and pregnant patients.
  • Pregnancy can lower the hematocrit by 10%.


CBC Tips and Notation – Simple Explanation of CNC Interpretation


References and Further Reading

  • Full Blood Count. Labsonline.uk Available from http://labtestsonline.org.uk/understanding/analytes/fbc/tab/test#how Accessed April 27, 2016.
  • Dunelavy R, Haematological Emergencies. In: Curtis K, Ramsden C. Emergency, and Trauma Care: for Nurses and Paramedics. Marrickville: Elsevier; 2011
  • Hematologic System Diseases and Disorders. In: Myers WM, Neighbors MM, Tannehill-Jones R. Principles of Pathophysiology and Emergency Medical Care. Clifton Park: Cengage Learning, Inc; 2002:313-314.
  • Full Blood Count. Patient.info Available from http://patient.info/doctor/full-blood-count#ref-1 Accessed April 27, 2016.
  • Murphy MF, Wainscoat JS, Pasi KJ. Haematological disease. In: Kumar P, Clark LM. Clinical Medicine 8ed. London: Elsevier Health Sciences, 2012:371-424
  • Nayak HN. Guidelines to Practice of Emergency Medicine 2ed. New Delhi: Elsevier India, 2008:72-75
  • Ballester JM. Appendix F: Interpretation of emergency laboratories. In: Mahadevan VS, Garmel GM, An Introduction to Clinical Emergency Medicine: Guide for Practitioners in the Emergency Department. Cambridge: Cambridge University Press, 2005:739-740
  • Full Blood Count: The Test – Lab Tests Online, Available from http://labtestsonline.org.uk/understanding/analytes/fbc/tab/test Accessed July 25, 2016.
  • Blood Count Explained – healthoracle.org, Available from http://www.healthoracle.org/downloads/C/Complete%20Blood%20Count%20Explained.pdf Accessed July 25, 2016.
  • Emblad PW. Laboratory Medicine. In: Mitchell E, Medzon R. Introduction to Emergency Medicine. Philadelphia: Lippincott Williams and Wilkins, 2005:521-523

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