Q&A: Diseases and Peripheral Blood Smear

Examination of the peripheral blood smear is an inexpensive but powerful diagnostic tool in both children and adults. In some ways it is becoming a "lost art" but it often provides rapid, reliable access to information about a variety of hematologic disorders. The smear offers a window into the functional status of the bone marrow, the factory producing all blood elements. It is particularly important when assessing cytopenic states (eg, anemia, leukopenia, thrombocytopenia). Review of the smear is an important adjunct to other clinical data; in some cases, the peripheral smear alone is sufficient to establish a diagnosis.

This article is for Medical Students & Professionals

This is a Question & Answer revision article designed for medical students and professionals preparing for the PLAB, MRCP or USMLE examinations. They are based on actual questions from these examinations. You may find the Blood article more useful, or one of our many articles on Diseases & Conditions, Medical Syndromes, Health & Wellness or Home Remedies.

In this article:
Review of peripheral blood smear
MCQ exam: clinical scenario
MCQ exam: answer
MCQ exam: explanation

Review of peripheral blood smear

Review of the peripheral smear is not required in all patients with a hematological disorder. Certain straightforward conditions such as iron deficiency anemia can be easily diagnosed on the basis of clinical information and basic laboratory data (eg, mean corpuscular volume [MCV], serum ferritin) alone. However, there are a number of settings in which interpretation of the peripheral smear is especially important. Three examples include:

• Hemolytic anemia – Review of red cell morphology may identify the cause of erythrocyte destruction (eg, the presence of bite cells points to a Heinz body hemolytic anemia) and the ultimate diagnosis (eg, oxidant damage to the red cell secondary to drugs).
• Thrombocytopenia – Review of platelet size and morphology can sometimes suggest whether thrombocytopenia is due to increased platelet consumption (generally associated with larger platelets) and reduced platelet production (often associated with smaller platelets or abnormal platelet morphology).
• White blood cell disorders – The precise disease classification may rely upon evaluation of abnormal circulating cells (eg, the presence of Auer rods in a blast form in patients with acute myeloid leukemia).

A normal peripheral smear should contain a spectrum of mature leukocytes including lymphocytes (30 to 40 percent of the circulating white cells), neutrophils, and monocytes. Eosinophils (less than 5 percent of white cells) and basophils (less than 1 percent of the total white blood cell count) are also encountered.

MCQ exam: clinical scenario

A 45 year old visits her physician because of respiratory complaints. After analyzing her blood work, she is told that she has a neutrophilic leukocytosis.

Which of the following is associated with neutrophilic leukocytosis?

a) Hayfever
b) Asthma
c) Acute bacterial infection
d) Ascaris
e) Loffler's Syndrome

MCQ exam: answer

The correct answer is C.

MCQ exam: explanation

A leukocytosis is an increase in the number of circulating leukocytes. A neutrophilic leukocytosis (neutrophilia) typically occurs in response to some sort of infection. All of the other conditions listed are associated with eosiniophilia.

Typically, acute bacterial infections are not associated with an eosinophilia. The most common causes of eosinophilia are allergic diseases and parasitic infections. The parasitic infections which are associated with eosinophilia involve invasion of the tissues. In addition, there are several other causes of eosinophilia.

The mnemonic for eosinophilia is 'worms, wheezes, and weird diseases'.

Hayfever and asthma both are due to a type I immune injury (IgE mediated), in which eosinophilia occurs. Ascaris is a parasitic tissue infection (non respiratory). 'Weird diseases' refers to a wide array of other conditions, including Hodgkin's disease, eosinophilic gastroenteritis, Loffler's syndrome and others (this is not an exhaustive list).

Reference(s)
1). UpToDate: Evaluation of the peripheral blood smear. Accessed ..20. Available online: https://www.uptodate.com/contents/evaluation-of-the-peripheral-blood-smear