Interpretation of the full blood count in systemic disease--a guide for the physician

The full blood count (FBC) is perhaps the single most common investigation performed in medical patients. It has the potential, when interpreted carefully and in relation to the clinical history, to provide very useful information to assist in diagnosis and management. Clinicians are often alerted to the presence of a primary haematological disorder by abnormalities in the FBC. For the purpose of this review these diseases will not be discussed in detail but the reader will be alerted to pointers which might indicate primary blood disorders throughout the text. The haematology laboratory in large teaching hospitals will often provide up to 1,500 automated FBC analyses each day. These are individually checked for 'flags' provided by the analyser which indicate values outside the normal range. It is clearly essential that clinical information is provided with the request as this will influence how the result is handled by scientific and medical staff. Furthermore, significant abnormalities will generate a blood film request and the report will be most useful when interpreted in light of the patient's working diagnosis. In cases where a diagnosis is not yet known, even brief information on presentation, for example 'collapse with hypotension', 'fever on return to UK', 'weight loss and anorexia', can all be important and help the lab provide clinicians with guidance. This short review aims to provide physicians with a workable guide to the interpretation of some of the commoner findings in the full blood count. Some of these will be very familiar to you but some will not. This review is not meant to be exhaustive as the rare minutiae will obscure the essential core material. Your haematology colleagues are always happy to help and available for assistance in difficult or problematic cases. I have not specified normal ranges in relation to each entity as these will be defined by your local laboratory.