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The hematology branch of clinical laboratory performs the testing on the blood cells, i.e. red blood cells (erythrocytes), white blood cells (leukocytes), and the platelets (thrombocytes). Bone marrow, lymph nodes, and the spleen are the main source of blood cells.

Cells and their constituents, their number or concentration, the relative distribution of different cells, their structural or biochemical abnormalities that contribute to the disease are measured by hematological tests.

Hematological analysis can be diagnostic or it could be the main contribution to the solution of a diagnostic problem. Many diseases have hematological manifestations and some diseases have a primary hematological cause. Many diseases produce anemia and enlarged lymph nodes. Several hematologic tests are basic to the initial evaluation and follow-up of the patient e.g. CBC, complete blood count which refers to the hematology profile generated by the instrument in use.

Skin puncture or venipuncture is the primary method collection of specimens for hematology and it is important to show cordial approach to the patient in general.
Capillary blood
Capillary blood collection can be used for many tests including hemoglobin, microhematocrit, red cell, white cell, and platelet counts. Capillary blood can be also used for the preparation of blood smears for microscopic analysis. It is important not to touch the sampling device to the skin when the tip of the sampling device is placed in the drop of blood when collecting capillary blood.

Collection of capillary blood can be done using the sampling device like micropipette, microcuvette, or a glass slide. An important criterion to be followed is the correct order of collection. The recommended order states that blood for hemoglobin test is drawn first, and then cell counts, and last is the preparation of blood films. When ordered, it is mandatory to do Platelet counts first. Pre analytical techniques like adequate mixing of blood with diluting fluid and knowledge of correct collection techniques is important for precise results.

Venous blood
Venous blood sample can be used to do erythrocytes sedimentation rate or ESR, requiring bigger volume of blood and almost all hematological tests. Ethylenediaminetetraacetic acid (EDTA) is the most common anticoagulant used to prevent clotting and to preserve cellular morphology.

Blood cannot be used for hematological tests even if it has small clots, and thus should be mixed with anticoagulant by gentle inversion 5-10 times after it is collected. If blood is collected in EDTA and tests are not done immediately, it can be used for White cell counts, microhematocrit, platelet counts, and sedimentation rates for upto 24 hours if it is refrigerated at 4 degree centigrade.

After collection, the specimen should be properly labeled and transported to the laboratory for analysis as quickly as possible for analysis. Standard precautions are complied during collection, laboratory handling, and testing using blood specimens. All samples are to be considered as potentially infectious, and the proper use of barrier-protective apparel and devices is mandatory.

Written by Phlebotomy Training specialist Dr Shahbaz A. Cheema, Course Director for Maxis Healthcare who run NHS Accredited Phlebotomy Training courses for medical and non medical practitioners. Learn the 3 Steps To Become a Phlebotomist