What’s in a Differential?

When a complete blood count (CBC) and differential is ordered by a physician, most labs today have instrumentation capable of performing an automated differential. Depending on the instrument results and flags, we may need to perform a scan, review of the slide, or a manual differential. However, the definition of a manual differential today may be a bit different than the historical definition. A typical manual differential, when I first started working as a technologist, consisted of counting and differentiating 100 white blood cells under a microscope, and performing a red blood cell morphology along with a platelet estimate. Today, the 3 components of the manual differential have not changed, but more and more  labs are using an automated digital counting device, such as CellaVision. Whether counting cells under the microscope or scanning and verifying or reclassifying cells in CellaVision, it is important to always address all 3 parts of the manual review.

When an automated CBC has flagged that abnormal RBC morphology may be present, a peripheral blood smear should be reviewed. Reporting the red blood cell (RBC) morphology is an important component of a differential. Evaluation and interpretation of RBC morphology may provide the physician with important diagnostic information regarding the underlying cause of a variety of disorders, including anemia and systemic disease. Therefore, it is important to be able to accurately recognize and identify RBC morphologic abnormalities.

Red blood cell morphology can be subjective, and therefore inconsistent. Therefore, Laboratories must have training and competency programs as well as  procedures which dictate how they will report RBC morphology. Some labs use a numbering system, 1+, 2+, 3+, and others report, ‘rare’, ‘few’, moderate’ or ‘many’. Some morphological, such as rouleaux, can just be reported as present, with no quantified. Any method is acceptable, as long as there is consistency in reporting.

When performing RBC morphology,  these semi-quantitative report formats for should be based on clinical significance. Some RBC morphologies and inclusions are clinically significant,even when they are present in very low numbers. Sickle cells are one of these abnormalities that are significant even if only seen in very small numbers. Malaria or other parasites are clinically significant in any number. Fragmented cells such as schistocytes and helmet cells should also be noted if seen in any number. Other abnormalities which can be clinically significant in very low numbers are polychromasia, spherocytes and teardrop cells.

There are many other abnormal RBC morphologies which are only clinically significant if seen in larger numbers. Laboratories may choose to only report the presence of ovalocytes, target cells, burr cells, macrocytes, microcytes or hypochromia when greater than a defined percentage of cells exhibit these morphologies. Other laboratories choose to not report macrocytes, microcytes and hypochromia at all, instead relying on the physician to use the RBC indicies for their indication. The 2 most important things to remember, whatever your procedures are, is to be consistent, and not to ignore the:RBC morphology.

In addition to performing RBC morphology, a manual differential also requires platelet examination. A smear should be examined for a platelet estimate and abnormalities. This is particularly important when platelet clumps or an abnormal platelet scattergram are flagged on the CBC.  If an instrument uses optical platelet counts, large platelets can be missed. A fluorescent platelet count (PLT-F) , performed on Sysmex analyzers, will stain only platelets and give an accurate platelet count. The fluorescent count eliminates interferences seen with other methods. However, even when reporting a PLT-F, it may still required to review the smear for a platelet estimate, particularly with a very low count, or with clumped platelet flags. Clumped platelets are not an uncommon phenomenon, and an accurate platelet count can not be reported if significant clumping is present. The presence of giant platelets or hypogranular platelets, seen on the slide,  can also aid the physician in diagnosis or patient management.

CellaVision users have the added benefit of automation which simplifies the process of performing manual differentials. The system automatically locates and takes digital images of cells, including white blood cells, red blood cells and platelets.This simplifies the process of performing a manual differential. White  blood cells are pre-classified, RBC images are provided, and platelet images allow platelet estimates to be performed easily. The new advanced RBC application software can pre-classify RBCs.  This makes it even easier than before to perform reliable, standardized RBC morphology. (Watch for my next Hematology blog about the new RBC software!)

Particular disorders or abnormalities often involve characteristic changes to RBC morphology “Assessment of RBC morphology can be the best tool for laboratory hematology professionals to recommend clinical and laboratory follow‐up in a patient with anemia and to select the right tests for definitive diagnosis.”1 Too often, I have seen technologists perform a manual differential and either superficially skim over the RBC and platelet components, or totally forget them. Don’t forget your RBC morphology and platelet estimate and morphology! With today’s automated differential and autovalidation, 75-85% of CBCs are autovalidated. This allows us to spend quality time on those manual reviews that need to be done. Be sure to spend your time thoroughly reviewing the slides. A scan, slide review or manual differential, whether done under the scope of with CellaVision,tells the physician that we have looked at the slide or cells, which must include all 3 parts of manual review… WBCs , RBCs and platelets. Don’t sign it out until it’s complete!

References

  1. J. Ford, Red Blood Cell Morphology. International Journal of Laboratory Hematology. 2013
  2. https://www.labce.com/red-cell-morphology.aspx

-Becky Socha, MS, MLS(ASCP)CM BB CM graduated from Merrimack College in N. Andover, Massachusetts with a BS in Medical Technology and completed her MS in Clinical Laboratory Sciences at the University of Massachusetts, Lowell. She has worked as a Medical Technologist for over 30 years. She’s worked in all areas of the clinical laboratory, but has a special interest in Hematology and Blood Banking. When she’s not busy being a mad scientist, she can be found outside riding her bicycle.

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