International Journal of Advanced Medical and Health Research

: 2022  |  Volume : 9  |  Issue : 1  |  Page : 1--3

In the era of automation and molecular techniques, is peripheral blood smear examination getting redundant?

Debdatta Basu 
 Department of Pathology, JIPMER, Puducherry, India

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Dr. Debdatta Basu
Department of Pathology, JIPMER, Puducherry - 605 006

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Basu D. In the era of automation and molecular techniques, is peripheral blood smear examination getting redundant?.Int J Adv Med Health Res 2022;9:1-3

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Basu D. In the era of automation and molecular techniques, is peripheral blood smear examination getting redundant?. Int J Adv Med Health Res [serial online] 2022 [cited 2022 Aug 8 ];9:1-3
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With the advances in automation in blood cell analyzers, the number of blood samples needing peripheral blood smear examination has decreased over the years.[1] Sophisticated cell counters provide not just counts but also differentials, scatter plots, and histograms, all of which aid in diagnostics with a much shorter turnaround time and lesser consumption of human resources.[2] The manual blood smear examination, which is still a very important diagnostic tool, is recommended to validate, and not replace, automated methods.[3] The examination is usually performed by an experienced and trained, medically qualified hematologist or pathologist. Hence, when compared to automated blood counts, blood smear examination needs expertise, is labor intensive, and must be used judiciously.

Requests for blood smear examination can be made at the laboratory end or by the physicians. At the laboratory end, smears are made either in response to an abnormal count or due to “flags” generated by an automated instrument. “Flags” are machine-generated messages when the values are not in the reference ranges and the machine suggests the possible changes, a type of artificial intelligence.[3] Making a smear from the same blood sample also means avoiding taking another blood sample just for this purpose. Consensus criteria for laboratory-initiated review of peripheral blood smears based on the results of the automated blood count have been published by the International Society for Laboratory Hematology (available at[4] Based on these guidelines, protocols for examination of blood smears must be there in all laboratories. It constitutes about 10%–15% of the total number of samples received in the laboratory.[1] In general, when all parameters, including counts, differentials, histogram, and scatter plots, are normal in the cell counter, a peripheral smear examination is not necessary.[2]

Often, a physician initiates a request for a blood smear. This is usually based on certain clinical features or on an abnormality shown in a previous complete blood count.[1] Some clinical situations that merit a blood smear examination are:

Anemia and/or unexplained jaundice raising doubts of hemolytic anemiaSickle cell disease – limb, chest, or abdominal pain with anemia and jaundice, especially in childrenPetechiae and other mucocutaneous bleeds suggestive of thrombocytopeniaUnexpected or severe infection and fever suggesting neutropeniaLymphadenopathy, splenomegaly, or a mediastinal mass on radiology suggesting a lymphoproliferative disorderSplenomegaly, ruddy cyanosis, itching, bone pain, or weight loss indicating a myeloproliferative disorderFeatures of disseminated intravascular coagulationRecent-onset renal failure, particularly in a childSuspicion of infections with hemoparasites such as malaria or filaria.

With regard to anemia, automated cell counters can provide valuable information. Apart from red cell count and red cell indices, they provide newer variables with information that can predict certain specific blood smear findings.[3],[4] These include red cell distribution width, which refers to the degree of anisocytosis, and hemoglobin distribution width which refers to the percentages of hypochromic and hyperchromic cells. Increased numbers of hyperchromic cells on a cell counter reflect spherocytes. Large normochromic cells could mean true macrocytes whereas hypochromic macrocytes could mean dual deficiency anemia or reticulocytes. Histograms and scatter plots also give a visual representation of features of red blood cells (RBCs). Despite this abundance of information, there are many morphologic abnormalities that can be demonstrated only in the peripheral blood smear examination and which are crucial in differentiating causes of anemia.[1] Some such important clinical situations are detailed below.

Schistocytes or fragmented red cell indicating microangiopathic hemolytic anemia (MAHA) is clinically very significant. Schistocytes are seen in hemolytic–uremic syndrome or thrombotic thrombocytopenic purpura; two clinical scenarios necessitating urgent diagnosis and management. MAHA is also seen in pregnancy-associated hypertension and disseminated intravascular coagulation. In MAHA, examining the blood smear helps to countercheck the platelet count, since fragmented RBCs and platelets may have very similar sizes and automated cell counters may not be able to differentiate the two.[1],[4],[5]

Spherocytes, although not specific, may be seen in hereditary spherocytosis, autoimmune hemolytic anemia, hemolytic disease of the newborn, or a delayed transfusion reaction. However, the presence of spherocytes in the peripheral blood smear, along with clinical features and the results of Coombs test, generally points to the correct diagnosis.[1] Microspherocytes are red cells that are small in size, hyperchromic, and are characteristic of burns and MAHA.

Peripheral blood smear examination is crucial in the diagnosis of acute hemolysis due to oxidant damage. The presence of “bite” cells and “blister” cells are characteristic. Oxidant-induced hemolysis is seen in glucose-6-phosphate dehydrogenase (G6PD) deficiency or when normal G6PD levels get overwhelmed by a very severe oxidant exposure. G6PD deficiency is common in India. There are two reasons why a blood smear is important in this scenario. First, the results are available far more quickly than the results of a G6PD assay permitting a provisional diagnosis and initiation of therapy. Second, a blood smear can pick up G6PD deficiency even if the assay is normal. This is because immediately after an episode of acute hemolysis, reticulocytosis (G6PD levels are higher in reticulocytes than mature red cells) may falsely result in normal G6PD levels. In such a circumstance, presence of bite cells in blood smear clinch the diagnosis and the assay may be repeated once the acute hemolytic episode is over.[1],[4]

In hereditary elliptocytosis, the blood smear is so distinctive that no other test is needed for diagnosis.[1] Red cell agglutinates or rouleaux formation usually indicates cold agglutinins or hypergammaglobulinemia, respectively. Detection of red cell inclusion often gives a clue to the diagnosis. Howell–Jolly bodies, which are nuclear fragments, are an important indicator of functional hyposplenism; in contrast, the absence of Howell–Jolly bodies and other changes of splenectomy in the smear from a patient who has undergone splenectomy, may indicate the presence of a residual splenic tissue, like an accessory spleen. Pappenheimer bodies, which are hemosiderin-containing granules, and basophilic stippling which reflect altered ribosomes are subtle clues which aid in the diagnosis of sideroblastic anemia, thalassemia, and lead poisoning. In emergency situations, especially when a child presents with features of jaundice and sickling, and other crises, detecting sickle cells is often very useful.[1]

Examination of the blood smear is useful when there is macrocytic anemia, reflected by a high mean corpuscular volume (MCV) in the cell counter. Deficiency of Vitamin B12 or folic acid manifests with oval-shaped macrocytes and hypersegmented neutrophils in the peripheral blood smear. Although serum Vitamin B12 and folic acid assays are available these days, the blood smear can provide a provisional diagnosis and appropriate treatment in severely anemic patients while awaiting results of the assays or in those who cannot afford the costly assays. In elderly patients, an important cause of macrocytosis is myelodysplastic syndrome (MDS). Features such as hypogranular or hypolobated neutrophils (Pelger–Huet cells), occasional blasts, and large or hypogranular platelets point to MDS. MCV may also be high because of hemolysis or acute blood loss due to the ensuing reticulocytosis; in that case, the blood smear shows many polychromatophils.[1],[5] In microcytic anemia, examination of the blood smear is, in general, less important. Red cell indices and serum ferritin levels, interpreted in the clinical context, allow diagnoses in most cases. However, the presence of target cells and basophilic stippling are clues to thalassemia trait than iron-deficiency anemia. The presence of teardrop cells along with nucleated RBCs is an indicator of fibrosis in the marrow resulting in a leukoerythroblastic blood picture in myelophthisic anemia.

In the context of white blood cells, blood smears must always be examined in unexplained leukocytosis or leukopenia. Whether it is lymphocytosis, monocytosis, or when the flags are raised suggesting the presence of blasts, examination of smear is mandatory. The presence of many nucleated red cells often increases the leukocyte count reported by the counter, necessitating correction. The variations from normal white blood count (WBC) scatter plot are a good indicator of the presence of an abnormal WBC in the peripheral smear, necessitating morphological evaluation.[2] Toxic granules in the cytoplasm of neutrophils are often an indicator of ongoing bacterial infection.[2] Low counts may suggest aplastic anemia, hairy cell leukemia, subleukemic leukemia, or bone marrow infiltration. Peripheral blood examination in leukemia and lymphoma provides a morphologic basis for rational use of immunophenotyping and other sophisticated investigations. Detection of Auer rods in blast cells is an indicator of its myeloid nature. Examination of blood smear in acute promyelocytic leukemia with faggot cells with stacks of Auer rods and in Burkitt lymphoma with its deeply basophilic and vacuolated cytoplasm, is of particular significance, because it facilitates not just rapid diagnosis but also institution of specific treatment, failing which both the diseases can be life threatening.[1]

A blood smear should always be examined in low platelet counts, both to confirm the thrombocytopenia and to look for underlying causes. Falsely reduced platelet counts can be due to the presence of small clots, platelet clumps, and satellitism or abnormally large platelets.[1],[3],[5] Immature and regenerating platelets are large and the counters, especially those relying on impedance method of counting, count them as RBCs. This creates a falsely lowered count in critical situations where immature platelets are seen as in recovery following chemotherapy or dengue fever or idiopathic thrombocytopenic purpura. Ethylenediaminetetraacetic acid-induced pseudothrombocytopenia, often causes low counts on the cell counter, but the smear shows large clumps of platelets. Fibrin strands in the smear indicate that the thrombocytopenia is false and due to clots in the sample. Underlying causes of thrombocytopenia that may be picked up in the blood smear include MAHA and acute leukemia.[1] Similarly, thrombocytosis should be confirmed microscopically with a blood smear; spurious high counts may be due to fragmented red cells in MAHA or fragments of blasts as can be seen in tumor lysis syndromes.

Morphological assessment of peripheral smear needs training, experience, and time. In this era of advancements and automation, there is an ever-increasing pressure on laboratory services to improve the quality and speed of reporting. It is important to reduce workload judiciously and improve turnaround time. At the same time, important diagnostic information must not be missed by entirely relying on automated cell counters. As seen above, automation cannot provide all the answers that are needed for the physician. Sometimes, it is possible for a precise and quick diagnosis to be made only from a blood smear, as in detecting malaria. Physicians should order a blood smear only when there are reasonable clinical indications and pathologists should examine a blood smear whenever the results of the complete blood count indicate the need for validation or for further workup of a detected abnormality. Even in the age of automation and molecular analysis, peripheral blood smear remains an important diagnostic tool.


1Bain BJ. Diagnosis from the blood smear. N Engl J Med 2005;353:498-507.
2Gupta T, Basu, D. Utility of scatterplot patterns of automated hematology analysers in white blood cell disorders – A comparative study with peripheral blood smears. Asian J Med Sci 2020;11:29-34.
3Asad S, Ahmed I, Ali N. Utility of peripheral film findings and its correlation with automated analyzer – An audit from tertiary care hospital. J Lab Physicians 2017;9:1-4.
4Barnes PW, McFadden SL, Machin SJ, Simson E. The international consensus group for hematology review: Suggested criteria for action following automated CBC and WBC differential analysis. Lab Hematol 2005;11:83-90.
5Jain M, Dolai TK. ABC of the peripheral smear. J Indian Med Assoc 2020; 118:19-23.