In patient care, diagnostic formulations rest on a tripod consisting of clinical history, physical examination and laboratory investigations. The Literature reveals that as much as 70% of clinical decisions and diagnoses are supported by laboratory medicine. Peripheral blood film (PBF) is a basic and a highly informative hematological tool at the clinician’s disposal in screening, diagnosis and monitoring of disease progression and therapeutic response. An adept understanding of peripheral blood interpretation is important for a successful clinical practice.
The diagnostic relevance of a PBF is enormous. The PBF exposes the morphology of peripheral blood cells, which ensures its place in the morphologic diagnosis of various primary and secondary blood and blood related diseases. It’s diagnostic relevance has not been lessened by advances in hematology automation and molecular techniques.
The laboratory may initiate peripheral blood film based on abnormal findings from an automated count or patients clinical information whose diagnosis may be supported by a peripheral blood film. The latter is guided by individual laboratory policies or local regulating guidelines.
To ensure accurate and reliable results, pre-analytical variables that can affect the quality of film must be controlled. These include patient preparation and consent, blood sampling technique, transport to the laboratory and sample preservation. Blood sampling is invasive therefore the patient/client should be counselled on the procedure. Slide preparation is done by trained personnel preferably a medical laboratory technologist, who can ensure quality slides for microscopy. Laboratory assistants can also be trained in the art of slide preparation. Slide preparation can be quite laborious especially if large numbers of specimens are to be handled reason why automated slide maker stainer are now more and more commonly used in order to standardize blood film (not technician dependent).
The cytologist should be a trained laboratory technologist but preferably a laboratory physician especially for slides with significant pathology. The slide is viewed at the body of the smear, usually beginning about one millimeter away from the tail (the monolayer part). The head of the smear should be avoided as the cell density is twice that seen at the tail. The head portion of the blood film might be of interest when investigating for presence of malaria parasites or microfilaria. The feathered end may be examined for platelet clumps and large cells like monocytes and blasts.
Microscopy requires a skilled systematic approach. A quick assessment of a smear can be made within 3 minutes but an abnormal film would require longer time for wider view and differential cell counts. Morphology of the blood cells on a PBF smear is best discussed in line with each hemopoietic cell lineage. The distribution, size, shape, color, cellular inclusions of the red blood cell (RBC) and morphology of the other major cell lines should be carefully assessed.
When laboratory results are generated, they must be transcribed into reports and signed by the hematologist especially when there is a significant PBF abnormality. The typical reporting format begins with the patient’s bio-data, hospital number, requesting physician, date of request, date of report and clinical summary/details of the patient. The body of the report includes detailed characterization of each of the major hemopoietic cell lines: erythrocytes, leucocytes and the platelets. This is followed by a summary of the significant findings, likely diagnosis with differentials, other recommended laboratory evaluations and authorizing signature of the laboratory physician with date.