Blood Smear: The Fifth Vital Sign in Hematology

Hematology is a fascinating field of science. Blood disorders can manifest as clinical signs and symptoms that can be similar to many other medical conditions, hence the beauty of incorporating the patient’s medical history and physical examination to reach an accurate diagnosis. While in general we have four vital signs, in hematology, examining the peripheral blood smear is the fifth one. Similar to the role of other vital signs, a normal peripheral blood smear is as helpful as an abnormal one in the journey to establishing a diagnosis.

Prior to starting a hematology and oncology fellowship, I did not know how to look at a blood smear, where or how to get one, or what to do with it. I remember pictures of blood smears along with buzz words of different cell names used to indicate specific diseases commonly tested in exams. In fellowship, I learned that a blood smear can assist my argument toward a correct diagnosis and can guide further workup and management.

Blood smears are usually done by lab technicians. Checking the quality of the blood smear is essential; I personally have few steps I go over prior to examination when I look at the slide under the microscope:

Check patient identifiers: Looking at a smear for a different patient may result in misdiagnosis and subsequent inappropriate testing.
Make sure the smear was done within two to three hours of blood collection: This usually results in the best staining which will be needed for proper interpretation.
Look at the slide and grossly check the smear, making sure it has two edges: These two edges include the feathered edge (a thin portion) and the thick edge (a base). Approximately two-thirds to three-fourths of the slide length should be covered by the smear with no irregularities, streaks, or holes.

Subsequently, I use a multistep process depending on the specifics of the microscope used to examine the smear:

Low power objective (usually 10×): This will help to assess the overall quality of the smear, abnormal red blood cell (RBC) distribution (e.g., rouleaux), and/or rapid detection of large abnormal cells (e.g., blasts).
Intermediate power objective (usually 40× or 50×): Search for an area where the RBCs are evenly distributed and barely touching each other. A dry look and subsequently an oil-immersed look can help in quantifying white blood cells (WBCs) and platelets to see if they align with what is reported by the hematology analyzer. This can help in excluding platelet clumps as a cause of apparent thrombocytopenia and in identifying the presence of abnormal-looking WBCs or giant platelets.
High power objective (usually 100×): A closer look at the oil-immersed field can help in identifying morphologic abnormalities in WBCs (e.g., toxic granulations or Döhle bodies) and RBCs (e.g., Howell-Jolly bodies). Moreover, quantification methods to assess the manual blood counts can be done by examining multiple fields.

The best way to build and improve blood smear reading skills is practice. I will always remember Dr. Mark Udden, who taught me these skills at Baylor. At the end of each clinic, we sat down and went over all the patients’ smears, the normal ones and the abnormal ones, which made me realize the importance of examining numerous normal smears in order to accurately identify abnormalities.

One resource to seek visual learning is the ASH Image Bank, which contains thousands of smears with short descriptions submitted by hematologists. You can learn things bidirectionally: Suspect an abnormality and see how it actually looks, or learn how an abnormality looks and try to identify it in future patients’ smears.

You can save a life by identifying serious illnesses when reviewing a smear and acting promptly. It is hard when you start, but only gets easier with time as you build your skillset through experience and with the help of colleagues and mentors. Being skilled in reviewing blood smears is rewarding indeed! Always remember to check the fifth vital sign in hematology: the patient’s blood smear.