A hemogram contains all of the pertinent information required for assessment of hematopoiesis as well as a visual assessment of plasma appearance and measurement of total solids (an estimate of total protein) in plasma. The hemogram can be broken down into groups depending on what we would like to evaluate:
- Erythrogram: This includes counts of red blood cells (RBC) and RBC mass (hematocrit), hemoglobin concentration, RBC indices (cell size, hemoglobin content), RBC size variation (red cell distribution width or RDW), reticulocyte counts (percentage or absolute), nucleated RBC count (per 100 WBC or absolute), and RBC morphologic features, including presence of parasites.
- Leukogram: This includes a total white blood cell (WBC) count (corrected for nRBCs) and a differential cell count of the individual leukocytes (absolute counts, but some laboratories also provide percentage counts), and WBC morphologic features, including presence of parasites.
- Thrombogram: This includes results for platelet numbers and size (mean platelet volume) and an estimate of platelet numbers from a blood smear (to verify any provided counts and check for clumps). Some laboratories also provide results for platelet mass (platelet crit).
- Plasma: In some laboratories, a measurement of total solids by refractometer (often called total protein by refractometer or total protein-ref) and a visual assessment of plasma color (hemolysis, lipemia, icterus) is provided as part of the hemogram.
For creating a hemogram, we do the following tests:
- Measurement of cell counts: These are usually provided by automated analyzers.
- Red blood cell (RBC): Several different counts of mature RBC are provided including RBC mass (hematocrit [HCT] or packed cell volume [PCV], RBC count, and hemoglobin concentration. Also, in anemic dogs and cats, immature anucleated RBC (reticulocytes) are also counted (percentage and absolute counts) to assess for a regeneration by the bone marrow in response to the anemia. Reticulocyte counts can be done in other species, but are not usually part of a routine hemogram.
- White blood cell (WBC): Total WBC count
- Platelets: Total count. Some analyzers also measure a plateletcrit (platelet mass), analogous to the HCT for RBC.
- Measurement of red blood cell and platelet indices: These is also provided by automated analyzers, although they can also be calculated manually for RBC from count measurements (HCT, RBC, Hgb). Indices are only done for RBC and platelets.
- RBC: Mean corpuscular volume (MCV, a measure of the average RBC size), mean corpuscular hemoglobin (MCH; a measure of the average hemoglobin content in individual RBC), mean corpuscular hemoglobin concentration (MCHC; the percentage of the RBC that consists of hemoglobin), RBC distribution width (RDW, a measure of the variation in RBC volume). These indices are usually only provided for mature RBC on a hemogram but some hematology analyzers (e.g. ADVIA 120, 2120) also yield similar indices for immature anucleate RBC (which is used by many clinical pathologists to facilitate interpretation of hemogram results, e.g. identification of iron deficiency).
- Platelet: Mean platelet volume (MPV, a measure of the average platelet size) is the main platelet index reported on hemogram results. Others that are yielded by automated analyzeres but not usually provided on hemograms,, mean platelet component (MPC, measure of the internal complexity of the platelet) and platelet distribution width (PDW, a measure of the variation in platelet volume).
- Blood smear examination: This encompasses examination of a blood smear prepared from peripheral blood and stained with a hematologic stain (e.g. Wright's stain or a rapid stain like Diff-quik). This yields the following information:
- Differential leukocyte count: This provides relative proportions of leukocytes in the blood and are converted to absolute counts by multiplying results by the total WBC count.
- RBC morphologic features: Changes in RBCs, including variations in RBC shape (poikilocytosis), hemoglobin content, presence of inclusions (e.g. iron) or pattern of arrangement (e.g. rouleaux or agglutination)
- WBC morphologic features and number: Changes in WBCs, including immature (toxic change) or abnormal (dysplastic) features, and presence of inclusions (e.g. iron). Total WBC count can also be estimated from a well made blood smear.
- Platelet morphologic features and number: Changes in platelet appearance, including degree of granulation, presence or absence of clumps. Platelet counts are routinely estimated from a well made blood smear to verify any automated count.
- Infectious agents: The smear is reviewed for the presence of infectious agents in plasma (e.g. microfilaria), RBC (e.g. Mycoplasma) and WBC (e.g. Anaplasma morulae) and platelets (e.g. Anaplasma morulae).
- Assessment of plasma appearance and total solids:
- Plasma appearance: The plasma is visually examined for evidence of hemolysis, lipemia and icterus. This gives clues as to the presence of underlying disease. Hemolysis and lipemia also interfere with the assays used to measure several hemogram results (e.g. hemoglobin concentration) and these interferences must be considered when interpreting results (for more information, see related links on interferences).
- Total solids: This is an estimate of total protein in plasma.
Hematology test interpretation
Results of hematology tests provide information on the function of the bone marrow and yields clues or even diagnosis as to the presence of underlying disease. Hematology tests should always be interpreted with respect to what is known about the patient (signalment, history, clinical signs, results of other diagnostic testing) and should not be interpreted in isolation. Many factors other than disease influence the results of hematology tests. These factors
may be preanalytical, analytical and post-analytical.
- Preanalytical: These are variables associated with the patient, sample collection and sample handling. These generally affect the composition of the body fluid before analysis and can have a major impact on result interpretation. Some of these patient- or sample-related variables, such as hemolysis, icterus and lipemia can be semi-quantified (mild, moderate, marked) by visually assessing the sample, which is a routine part of most hemograms (see above).
- Analytical: These are factors which influence the analytical procedure, such as precision and accuracy.
- Post-analytical variables: This involves the different ways data from the laboratory is presented, stored and transferred to the clinician.
Whenever possible, these variables should be controlled in order to minimize their effect on test outcome.
Results for hematology tests are generally interpreted in two ways:
- On an individual basis: Results for individual hematology tests are generally interpreted with respect to reference intervals. Reference intervals should be established for each analyzer, since they will vary between laboratories (based on the animal population and number of animals used to determine the intervals, analyzer and method used to measure blood cells). The degree of change above or below the upper or lower reference limit that is actionable (of diagnostic relevance) differs between analytes and is highly subjective (and a matter of opinion). Mild changes in one result (e.g. MCV) are less clinically relevant than others (e.g. hematocrit). Although we are clued into look at the abnormal "H" and "L" flags, sometimes normal results may actually be abnormal. For example, dehydration may increase RBC counts to within the reference interval in a mildly anemic animal. It cannot be over-emphasized that all laboratory data should be interpreted with respect to the patient (clinical signs, results of diagnostic imaging or other testing, suspected or known diagnoses). If the laboratory results do not fit with the patient, repeat testing (on a fresh sample) may be warranted. Also, remember that a hemogram is a snapshot in time. The bone marrow and blood cells respond rapidly (within minutes to hours) to different situations and repeat testing is worthwhile to look for progression or resolution of disease. Since changes occur so rapidly, repeat hemogram testing is usually done every 24 hours in critical patients.
- Grouping results (pattern recognition): Hematologic tests can be grouped together on the basis of cell type. Grouping tests by cell type enables pattern recognition. Patterns of change within and among cells can provide useful diagnostic information about various diseases. The following is a classic way of grouping hematologic test results:
- Erythrogram: All RBC results, including an assessment of the regenerative response (reticulocyte counts, if provided) and morphologic features.
Note: total solids are also examined along with the erythron to help interpret changes in RBC number. For example, blood loss will usually result in anemia (decreased RBC counts) and low total solids.
- Leukogram: All WBC results, including total and differential leukocyte count, and assessment of WBC morphologic features.
- Thrombogram: Platelet number and size. A blood smear examination with an estimate of platelet numbers, is crucial when interpreting these platelet results. If other screening hemostasis tests are available (only indicated in some animals), platelet numbers should be interpreted alongside those results (particularly when low).
- How to make a good blood smear: Information from the Clinical Pathology Laboratory in the Animal Health Diagnostic Center at Cornell University website