Complete Blood Count (CBC)(also called Full Blood Count (FBC)) is a series of tests used to evaluate the composition and concentration of cellular component of the blood. It is a basic, important, readily available and most informative single investigation. It can help to serve as a screening test and as a prognostic or follow-up tool. It includes red cell parameters (RBC, HGB, HCT, MCV, MCH, MCHC, RDW, RET); total and differential absolute white cell count; platelet parameters (PLT, MPV, PCT) and blood film report. Interpretation of CBC should be based on a combination of individual results of RBC, WBC and PLT parameters, histograms, scatter grams and blood film examination.
Red Cell Parameters
Anemia is defined as a decrease in RBC mass but in practice it is defined by haemoglobin (HGB) concentration. It is a state in which the level of HGB in blood is below the normal range appropriate for age and sex; generally for male, HGB is less than 13 g/dl, for female HGB less than 12 g/dl and for pregnant women less than 11 g/dl. HGB may be falsely raised by turbidity due to lipemia, precipitation of monoclonal proteins, very high white cell count, admixture of fetal blood and very high bilirubin level. HGB level is also spuriously low in improper sample mixing, blood taken from intravenous infusion site (dilution) and agglutination. Haematocrit (HCT or PCV) is the ratio of the volume of erythrocytes to that of the whole blood expressed as a fraction of the whole (HCT) or as a percentage (PCV). It is a more precise method for determining anemia or polycythaemia. There is the rule of three i.e. RBC x 3 = HGB; HGB x 3 = HCT. Causes of decreased or increased RBC/HCT are shown in table(1).
Mean Corpuscular Volume (MCV) is the average size of red blood cells. If anemia is present; MCV is a useful tool to guide further testing. If anemia is not present, MCV is of little value: low MCV without anemia suggests thalassemia minor (trait) and high MCV without anemia can be caused by certain medications (hydroxurea, methotrexate) and is a “soft” marker of possible alcohol overuse. Red Cell Distribution Width (RDW) measures the variability in the size and volume of red blood cells (anisocytosis) but is not useful in the absence of anemia. Increased RDW reveals mixed population of red cells (early nutritional deficiency, fragmentation, agglutination and dimorphic population). RDW is usually either normal or increased. A decreased RDW calls for a review. RDW-CV (coefficient of variation) is the ratio of SD to MCV (SD of RBC volume/mean MCVx100) and is the index of degree of anisocytosis in cell volume within the red cell population but it can be affected by MCV. RDW-SD (standard deviation) is calculating the width in fl at the 20% height level of red cell histogram and is the actual measurement of the width of RBC distribution curve. Differential diagnoses of anemia can also be classified according to MCV and RDW (Table. 2).
MCH is calculated by HGB/RBC x 100. MCH is decreased in microcytic and normocytic anemia but it is increased in macrocytic anemia and infants and newborn. MCH interference occurs in lipaemia, marked leucocytosis, cold agglutinin and paraproteinemia. MCHC is calculated by HGB/HCT x 10. MCHC is decreased in hypochromic microcytic anemia but it is increased in hereditary spherocytosis, autoagglutination, infants and newborn. MCHC interference occurs in marked leucocytosis, haemolysis, cold agglutination and rouleaux formation. Reticulocyte count (RET) show as % or absolute count (% of reticx patient’s HCT/ normal HCT). In the setting of anemia, low RET indicates a condition affecting the production of red blood cells and a high RET generally indicates peripheral cause.
White Cell Parameters (WBC)
Most counters cannot exclude giant platelet, nucleated RBC. Differential white cell counts are more precise than percentage. In 3 part differential (18 parameters) shows GR/NEUT (large), LYM (small), MXD (middle cell= Mono+Eo+Baso) and the 5 part (26 parameters) shows Neutrophil (N), Lymphocyte (L), Monocyte (M), Eosinophil (E) and Basophil (B). The 7 part extended differential can also reveal immature granulocytes (IG) and nucleated red blood cell (NRBC).Variation of white cell count is shown in table (3). The spurious result of leucocytosis can occur in platelet clumping and giant platelet, nucleated red cells, red cell resistance to lysis, cryoglobulin and cryofibrinogen.
Platelet Count (PLT) Platelets are the really more of fragments of a cell. They are broken off from megakaryocytes. Lifetime in the blood is 7-10 days after which they are destroyed in the spleen. Causes of thrombocytosis and thrombocytopenia are shown in Table (4).
PLT is spuriously high in many small fragmented red cells (schistocytes, severe iron deficiency anemia, burns), microcytic red cells, cytoplasmic fragments of nucleated cells (leukemia, lymphoma cells), cryoglobulins and cryofibrinogen, bacteria and fungi and electronic noise. PLT can also spuriously low (pseudo-thrombocytopenia) in platelet agglutination (EDTA, PLT aggregates enumerated as WBC), platelet satellitism (mainly related to EDTA) around polymorphs and around other white cells (normal or pathological), giant platelet (outside the normal range and enumerated together with WBC), coagulation within the sample, overfilling the sample (inadequate mixing). Mean platelet volume (MPV) is the “Young” platelets, recently released from the bone marrow, are typically slightly larger and often elevated in immune thrombocytopenic purpura (ITP). In an individual with thrombocytopenia: increased MPV indicates normal bone marrow response and decreased or low normal MPV may indicate impaired bone marrow response.
Histograms and Scattergram
Histograms are the graphical representation of numerical data of different cell population on cell counter. Y axis represents the number of cells and X axis represents the cell size. Scattergram or DOT plot have better abnormal cell detection. However, interpretation of these cannot be explained in this section.
Manual blood film examination
It is the essential part of CBC interpretation. Clinical information is needed to interpret the blood film result. Discussion of blood film is, however, beyond the scope of this article.
Verification of the CBC result includes:
+Cross checking: RBC / HGB / HCT (Rule of Three);
+ Checking of biochemistry data (lipidemia, hyperglycemia, bilirubinemia, etc);
+ Checking with patient’s symptoms and signs
+ Checking of specimen identification, sampling, machine QC and
+Examination of good peripheral blood film (provide clinical information to pathologist)
The Eye can see only those things which the brain knows. Data of CBC alone will not guarantee a specific diagnosis in every case. Every disease does not have a single pattern of abnormality of CBC. Proper interpretation of CBC can be useful in narrowing differential diagnoses and aid in avoiding unnecessary investigations. Manual blood film examination remains the definitive tool for complete hematological analysis. Complete Blood Count (CBC) provides the numbers that tell you a lot of patient’s information about their health. The patient’s symptoms and signs, peripheral blood film examination and other investigations must be evaluated to get the final diagnosis. Interpretation of the numerical data of CBC, in conjunction with the histograms, scatter plots and blood film report can be clinically useful for diagnosis and follow-up of many hematological and non-hematological conditions.
Suggested reading:
- Robert Pierre ‘Seminar and Case Studies: The Automated Differentials’, 1984, Coulter Corporation.
- K Sango, Y Kosaka, K Yamamoto and Y Jyokesi ‘Automated Hematology Analyzer: Clinical Case Report’, second edition, 2005, Sysmex Corporation.
- RaiMra, HtunLwinNyein and Aye AyeGyi ‘Numbers that Tell: The Automated Full Blood Count’ 2011, CME program book, Myanmar Medical Association, Yangon.
- HtunLwinNyein ‘How I evaluate the results of full blood count’, CME program book, Myanmar Society of Haematology, Nay PyiDaw.
- DP Lokwani ‘The ABC of CBC: Interpretation of Complete Blood Count and Histograms’, first edition, 2013, Jaypee.
HtunLwinNyein, Professor and Head, Department of Clinical Haematology, University of Medicine (1), Yangon,Yangon General Hospital
Htun Lwin Nyein, Khaing Shwe Wah Pwint and Swe Mar Linn
