Laboratory Medicine & Hematology & Family Medicine
Peripheral blood smear
The peripheral blood smear - also known as blood morphology or peripheral blood examination - is the microscopic study of circulating blood cells on a thin layer of blood smeared on a glass slide, stained with May-Grünwald-Giemsa (MGG) or Wright-Giemsa. It's one of the most informative and inexpensive laboratory tests in hematology - simultaneously assessing the morphology of red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes), providing irreplaceable qualitative information that modern hematology machines can't fully reproduce. While the automated blood count (CBC) accurately measures quantitative parameters (hemoglobin + GMV + MCHT + WBC count with automated differential + platelets), it is the smear taken and read by an experienced hematologist or medical technologist that identifies subtle morphological abnormalities pointing to specific diagnoses: schizocytes (thrombotic microangiopathy - TTP + HUS); sickle cells (sickle-cell anemia); Howell-Jolly bodies (asplenia); circulating blasts (acute leukemia); Sézary cells (cutaneous lymphoma); intraerythrocytic Plasmodium (malaria). The peripheral blood smear is prescribed as a complement to the CBC when the latter shows unexplained abnormalities, alarms on the automaton, or as part of the work-up of a known or suspected hematological pathology. In Quebec, it is performed in hospital hematology laboratories and accredited private laboratories, on medical prescription.
Indications, technique and reading
- Clinical indications for peripheral blood smears : formal indications (smear recommended systematically): unexplained anemia (Hb <100 gl chez la femme + <120 l'homme)surtout si microcytaire ou macrocytaire anomalie de nfs avec alarme sur l'automate (blastes cellules atypiques corps étrangers) pancytopénie (atteinte des trois lignées) thrombopénie isolée sévère (<50 l) hyperleucocytose (>30 G/L) or leukopenia (<2 gl) + suspicion de paludisme d'anémie hémolytique (ldh élevée haptoglobine basse bilirubine indirecte élevée) microangiopathie thrombotique (ptt shu — thrombopénie anémie hémolytique) splénomégalie inexpliquée polycythémie (hb>185 g/L male + >165 g/L female); follow-up indications (smear useful for monitoring): follow-up of known hematological malignancies (leukemias + lymphomas + myelomas) + evaluation of response to treatment (chemotherapy + radiotherapy + biotherapies) + post-splenectomy monitoring + follow-up of constitutional hemolytic anemias (sickle cell anemia + thalassemia + hereditary spherocytosis) + post-transfusion monitoring of hemolytic patients.
- Production techniques and reading zones : sampling: EDTA tube (purple cap - calcium-chelating anticoagulant) - smear ideally taken within one hour of sampling (rapid morphological degradation of leukocytes + changes in erythrocyte shape if delayed) - if delay unavoidable: store at +4°C for up to 24 hours; spreading technique: deposit of a drop of blood (2-4 µL) at one end of the slide + regular spreading with a second slide at 30-45° → uniform thin layer 3-4 cm long + decreasing thickness from head to tail - air drying + automated (laboratory) or manual MGG staining; microscopic reading zones: thick zone (head of smear): cells superimposed - not readable - not interpretable; body zone (monolayer): optimal reading zone - cells well spread out + no superimposition + preserved morphology - systematic reading with × 10 then × 40 then × 100 objective (immersion oil) - differential leukocyte count (100 leukocytes minimum) + evaluation of erythrocyte and platelet morphology; tail zone (reading head): stretched and distorted cells - frequent artifacts (false schizocytes + acanthocytes) - not to be interpreted apart from the search for blasts and parasites; May-Grünwald-Giemsa (MGG) staining: erythrocytes → salmon-pink; mononuclear leukocytes → violet/blue nuclei + blue-grey cytoplasm; granulocytes → specific granulations (neutrophils: pink-purple granulations + eosinophils: bright orange granulations + basophils: dark purple granulations)
- Parameters systematically evaluated on the smear : erythrocytes : size (normocytes + microcytes + macrocytes) + shape (poikilocytosis - sickle cells + schizocytes + acanthocytes + echinocytes + elliptocytes + spherocytes + stomatocytes + targets) + staining (normochromia + hypochromia + polychromasia = polychromatophilic reticulocytes) + inclusions (Howell-Jolly bodies + basophilic granulations + Heinz bodies = denatured hemoglobin + intraerythrocytic Plasmodium) + roll formation (erythrocyte rolls = high fibrinogen + myeloma) ; leukocytes : manual differential (100 cells : neutrophil polynuclears + eosinophils + basophils + monocytes + lymphocytes + others) + nuclei morphology (neutrophil hypersegmentation = B12/folate deficiency + Döhle bodies + toxic granulations = severe infection + blasts = leukemia + Sézary cells = lymphoma) + presence of immature cells (myeloma); platelets: estimated count (normal: 8-20 platelets per field × 10 = approx. × 15,000/mm³ per visible platelet) + morphology (macro-platelets + micro-platelets + platelet aggregates = false EDTA-dependent thrombocytopenia) + granularity
Morphological abnormalities and their clinical interpretation
| Morphological abnormality | Description and microscopic appearance | Main etiologies and clinical orientation |
|---|---|---|
| Morphological abnormalities of erythrocytes - microcytosis and hypochromia VGM <80 fL - TCMH <27 pg |
Microcytes: small erythrocytes (diameter <6 µm) - GMV 1/3 of erythrocyte diameter - MCHT <27 pg); appearance on smear depending on etiology: martial deficiency: microcytes + hypochromia + anisocytosis (size variation - high IDR) + mild poikilocytosis + rare target cells - progressive appearance according to severity of deficiency - at very advanced stage: ovalocytes + hypochromic elliptocytes; thalassemia minor (thalassemic trait α or β): marked microcytosis + hypochromia + target cells (codeocytes - flat discs with dense central zone surrounded by a pale halo = corona hemoglobin) + hypochromic erythrocytes + anisocytosis + poikilocytosis - normal or slightly increased IDR (contrast with martial deficiency where IDR is elevated); inflammatory anemia (chronic disease): moderate microcytosis (VGM rarely <70 fL) + normo or slight hypochromia - morphological appearance often not very specific + martial workup (high ferritin + low CST) guides the diagnosis; lead intoxication (lead poisoning): microcytes + hypochromia + intraerythrocytic punctate basophilic granulations (basophilic stippling - precipitation of ribosomes) + anisocytosis - in children exposed to lead paint + in adults in industrial environments. | Diagnostic approach to microcytosis on smear: martial workup (ferritin + serum iron + CST + TIBC) + hemoglobin electrophoresis (if thalassemia suspected - high HbA2 profile >3.5 % in β-thalassemia minor) + CRP (inflammation) + reticulocytes; martial deficiency (most common cause of hypochromic microcytosis): ferritin <30 µg/L + CST 50 years or male or digestive symptoms) + malabsorption (celiac disease - serology) + inadequate intakes; thalassemia minor: CBC with normal IDR + very low VGM for Hb (Mentzer VGM/GR ratio 13 → martial deficiency) + Hb electrophoresis - no iron treatment if pure thalassemia (risk of iron overload) - genetic counseling if pregnancy desired (risk of thalassemia major if both parents are carriers) |
| Macrocytosis and macrocytic morphological abnormalities VGM >100 fL - megaloblastosis + others |
Macrocytes: large erythrocytes (diameter >8.5 µm) - GMV >100 fL; megaloblastic macrocytosis (vitamin B12 or folate deficiency - antifolate drugs): macro-ovalocytes (large ovalocytes + well hemoglobinized - characteristic of megaloblastosis) + hypersegmentation of neutrophilic nuclei (≥5 lobes in >5 % neutrophils OR presence of a neutrophil with 6 or more lobes - early and very specific sign of megaloblastosis). early and very specific sign of B12/folate deficiency) + anisocytosis + poikilocytosis + rare teardrop erythrocytes (dacryocytes) - macrocytes are oval and not round (important distinction from non-megaloblastic macrocytes) ; non-megaloblastic macrocytosis (rounded macrocytes - without macro-ovalocytes or neutrophilic hypersegmentation) : alcoholism (frequent - direct mechanism on erythropoiesis + folic deficiency often associated - macrocytosis without severe anemia) + chronic liver disease (macrocytosis + acanthocytes = spur cells in advanced cirrhosis) + hypothyroidism + bone marrow aplasia (macrocytes + pancytopenia) + myelodysplastic syndrome (MDS - multiple dysplastic abnormalities + macrocytosis ± pseudo-Pelger-Huët hyposegmented neutrophils) + drugs: hydroxyurea + azathioprine + methotrexate + stavudine (ARV); marked reticulocytosis: pseudo-macrocytosis due to presence of polychromatophilic reticulocytes (larger than mature erythrocytes - occur during active bone marrow regeneration) | Assessment of macrocytosis : serum vitamin B12 (normal >200 pmol/L - sensitive but imperfect assay - a B12 in the low normal range 200-300 pmol/L may mask a deficiency - supplement with plasma methylmalonic acid + homocysteine if in doubt) + folates erythrocyte folates (reflect intracellular reserves - more reliable than serum folates) + TSH (hypothyroidism) + liver work-up + GGT (alcohol) + reticulocytes + myelogram if MDS suspected (pancytopenia + multiple dysplastic smear abnormalities); treatment of B12 deficiency : cyanocobalamin or hydroxocobalamin IM 1,000 µg × 1/d × 7d + 1,000 µg × 1/week × 4 wk + 1,000 µg × 1/month for life if irreversible cause (atrophic gastritis + gastrectomy + Biermer's disease) - or high-dose oral B12 1,000-2,000 µg/d if dietary cause and intact digestive mucosa (passive absorption) - expected response: maximum reticulocytosis at D7 + Hb rise of 10-20 g/L/month + normalization of VGM in 2-3 months; treatment of folate deficiency: folic acid 1-5 mg/d PO × 3-4 months (always be sure to exclude B12 deficiency before supplementing with folates alone - risk of aggravating B12 neuropathy if folates alone administered) |
| Schizocytes and microangiopathic hemolytic anemia Hematological emergency - PTT + SHU + CIVD |
Schizocytes (or schistocytes): erythrocyte fragments resulting from mechanical destruction of red blood cells in contact with intravascular fibrin filaments or abnormal surfaces (valve prostheses + vascular stenosis) - varied appearances: triangles + helmets + comma shapes + irregular fragments - always abnormal (pathological threshold: >0.1-0.2 % of erythrocytes according to ICSH 2012) - blood smear is the essential examination to detect them (automats do not identify them reliably); thrombotic microangiopathy (TMA) - diagnostic urgency: association schizocytes + thrombocytopenia + hemolytic anemia (very high LDH + collapsed haptoglobin + high indirect bilirubin) + negative direct coombs → confirmed TMA - 3 main entities to distinguish quickly as treatments are different: TTP (thrombotic thrombocytopenic purpura): severe ADAMTS13 deficiency (90 %; HUS (hemolytic uremic syndrome): MAT + dominant renal failure + few or no neurological manifestations - typical STEC HUS (E. coli O157:H7 - mostly children - after bloody diarrhoea) + atypical HUS (alternative complement pathway deficiency) - treatment: eculizumab (atypical HUS) + renal supportive care; DIC (see fibrinogen data sheet): schizocytes + thrombocytopenia + prolonged PT + low fibrinogen + very high D-dimer - treatment of cause | Approach to schizocytes on smear: ADAMTS13 assay as a matter of urgency (before plasma exchange if possible, but do not delay treatment if TTP highly likely) + direct coombs + LDH + haptoglobin + bilirubin + PT + APTT + fibrinogen + D-dimer + renal function + urine (proteinuria + haematuria) + complement panel (C3 + C4 + CH50 + factor H + factor I) if atypical HUS suspected; schizocytes outside MAT: defective or dysfunctional mechanical valve prosthesis (paravalvular leakage) + severe aortic stenosis (Waring Blender mechanical intravascular hemolytic anemia - high shear stress on erythrocytes) + severe PAH + aortic coarctation; schizocytes + thrombocytopenia following stem cell allograft : transplant-associated thrombotic microangiopathy (TA-TMA) - diagnosis of exclusion + eculizumab discussed; importance of schizocyte quantification: >1 % = probable MAT + >5 % = severe MAT - ICSH 2012 recommendation: count 1,000 erythrocytes on the body area of the smear + express the result as a percentage |
| Leukocyte abnormalities - blasts and abnormal cells Leukemia - lymphoma - severe infections |
Circulating blasts: immature myeloid or lymphoid cells in peripheral blood - normally absent in blood - their presence is a major alarm on the automaton + must always be confirmed by smear + reading by a hematologist; myeloid blasts (acute myeloid leukemia - AML): large irregular nuclei + fine, loose chromatin + prominent nucleoli (1-3) + basophilic cytoplasm + azurophilic granulations + Auer rods (cytoplasmic red rods pathognomonic of AML - especially present in AML-M2 and APL) - blast count >20 % → AML according to WHO 2022 classification; lymphoid blasts (acute lymphoblastic leukemia - ALL): small to large regular nuclei + condensed chromatin + inconspicuous nucleoli + basophilic cytoplasm without granulations + PAS positive - ALL-B (more frequent in children) vs. ALL-T + immunophenotyping by flow cytometry mandatory for classification; chronic lymphocytic leukemia (CLL): persistent lymphocytosis + small, mature lymphocytes with monomorphic appearance + Gumprecht shadows (spreading fragile lymphoid cells - pathognomonic of CLL) + rare lymphoplasmacytoid forms; lymphoma in leukemic phase (Sézary cells + villeous lymphocytes + mantle lymphocytes); leukemic reaction (non-malignant): reactional hyperleukocytosis + myelemia (immature non-blastic forms: metamyelocytes + myelocytes - without blasts >5 %) → severe infectious context + inflammation + Marshall syndrome; toxic granulations + Döhle bodies: coarse blue-black cytoplasmic granulations in neutrophils → sign of severe bacterial infection + sepsis + burns | Treatment of circulating blasts or suspected hyperleukocytosis: urgent hematology consultation - if blasts >20 % → acute leukemia → urgent hospitalization in hematology + complete workup (myelogram + cytogenetics + molecular biology) + prevention of tumor lysis syndrome (hyperhydration + allopurinol or rasburicase) + initiation of chemotherapy; leukostasis (acute leukemia with leukocytes >100 G/L): life-threatening emergency - encephalopathy + respiratory failure + infarction - emergency leukapheresis discussed + immediate chemotherapy; eosinophilia (eosinophilic polynuclei >0.5 G/L): reactive (allergy + parasitosis + asthma + IBD + drugs) or clonal (chronic eosinophilic leukemia - FIP1L1-PDGFRA mutation - treatment: imatinib + hypereosinophilic syndrome) - smear: mature eosinophils with characteristic orange granulations (reactive) vs. dysplastic eosinophils + immature precursors (clonal); severe neutropenia (<0.5 G/L - agranulocytosis): major infectious risk → smear to assess morphology + myelogram if persistent → causes: drug-induced (chemotherapy + clozapine + propylthiouracil + ticlopidine) + autoimmune + bone marrow aplasia + Felty syndrome (RA + splenomegaly + neutropenia) |
| Platelet abnormalities and intraerythrocyte parasites Thrombocytopenia - malaria - Babesia |
Platelet morphological abnormalities: macroplatelets (giant platelets - diameter >4 µm): Bernard-Soulier syndrome (congenital platelet von Willebrand disease) + myeloproliferation + reactive bone marrow regeneration + immune thrombocytopenia (TPIa - macroplatelets reflect compensatory bone marrow production); microplatelets: Wiskott-Aldrich (syndrome of) + consuming DIC; platelet aggregates: EDTA-induced platelet clumping (false thrombocytopenia - frequent - 0.1-1 % of EDTA samples) → repeat CBC on citrate tube + check smear for absence of true thrombocytopenia → thrombocytopenia is artifactual if aggregates are numerous and free platelets rare; reduced platelet granularity (gray platelets): gray platelet syndrome - NBEAL2 mutation - absence of alpha granules - rare constitutional hemorrhagic thrombopathy; intraerythrocytic parasites - diagnostic emergency in the context of tropical travel: Plasmodium falciparum (severe malaria): delicate intraerythrocytic rings + small trophozoites in marginal position (attached to the membrane) + infected erythrocytes not enlarged + crescent-shaped gametocytes (banana - mature forms) + variable parasitemia (sometimes very high >5 % in P. falciparum) - MEDICAL EMERGENCY; Plasmodium vivax/ovale: enlarged + deformed infected erythrocytes + Schüffner granulations + ring or amoeboid trophozoites + schizonts; Babesia microti: intraerythrocytic Maltese cross tetrad (pathognomonic of Babesiosis) + may mimic P. falciparum - DO NOT treat. falciparum - DO NOT treat as malaria - treatment: atovaquone + azithromycin (or quinine + clindamycin if severe); Trypanosoma (African trypanosomiasis - sleeping sickness): flagellate extracellular trypomastigotes visible between erythrocytes | Approach to thrombocytopenia on CBC: always check smear to rule out false EDTA thrombocytopenia (aggregates) - if true thrombocytopenia: look for schizocytes (MAT) + blasts (leukemia) + Gumprecht shadows (CLL) + erythrocyte morphological abnormalities (Evans syndrome = autoimmune hemolytic anemia + PTAI) + macroplatelets; primary immune thrombocytopenia (PIT - formerly PTI): isolated thrombocytopenia + smear: macro-platelets + no abnormalities in other lineages + no blasts + no schizocytes - diagnosis of exclusion - myelogram if >60 years or atypical signs - treatment : corticosteroids (prednisolone 1 mg/kg/d) + IVIG if bleeding emergency + eltrombopag/romiplostim if corticoresistant + rituximab + splenectomy; malaria and smear: reference examination - optimal sensitivity: 3 thin smears + 3 thick drops 12 hours apart to exclude the diagnosis - results expected within the hour in Quebec (university hospital laboratories - 24-hour on-call availability) - rapid antigen detection test (RDT): sensitivity 90-99 % for P. falciparum + less sensitive for other species + does not replace smear for parasitaemia quantification and treatment follow-up |
ℹ️
Peripheral blood smear vs. automated CBC - essential complementarity : automated CBC and peripheral blood smear are complementary, not interchangeable. The automated hematology system measures quantitative parameters (hemoglobin, GMV, cell count) with great precision, but cannot reliably identify qualitative morphological abnormalities - it generates alarms that need to be checked on the smear. In contrast, the smear is a semi-quantitative, qualitative examination that depends on the reader's expertise. In clinical practice in Quebec, the smear is performed automatically by laboratories when the CBC generates specific alarms (suspicious blasts + atypical cells + suspicious schizocytes + abnormal platelets). Physicians may also request it explicitly - the words «peripheral blood smear» or «blood morphology» on the prescription are sufficient.
Results requiring urgent medical assessment
Smear with circulating blasts + pancytopenia → probable acute leukemia → urgent hematology consultation the same day - don't wait.
Schizocytes >1 % + thrombocytopenia + hemolytic anemia → MAT (PTT or SHU) → urgent hospitalization - plasma exchange in PTT must be started within hours of diagnosis.
Intraerythrocytic parasites on return from tropical travel + fever → malaria → immediate medical emergencies - P. falciparum malaria can kill in less than 24 hours without treatment.
Consult at Clinique Omicron
Clinique Omicron's doctors prescribe and interpret peripheral blood smears as part of the work-up for anemia, abnormal CBC, return from a trip with fever, or suspected hemopathy. Abnormal results are rapidly referred to a hematologist or internist, depending on the diagnosis. Consultations are available at several points of service in Quebec and via telemedicine. To book an appointment, visit cliniqueomicron.ca.
The contents of this page are provided for information purposes only and do not replace the advice of a qualified healthcare professional. Interpretation of the peripheral blood smear requires expertise in morphological hematology and must always be correlated with the clinical context and other biological parameters.
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