Complete Blood Count (CBC) with Differential and Platelets
The Complete Blood Count with Differential and Platelets from Quest Diagnostics quantifies and characterises white blood cells (5-part absolute and percentage differential), red blood cells (with haemoglobin, haematocrit, MCV, MCH, MCHC, and RDW), and platelets (with mean platelet volume) from a single EDTA whole blood specimen.
3
Biomarkers
Blood (whole blood)
Sample
1–2 business days
Turnaround
No — fasting does not affect haematological parameters and is not required for the CBC. Pre-analytical specimen handling technique is the dominant determinant of result quality for this test.
Fasting
What is the CBC with Differential test?
The Quest Diagnostics CBC with Differential and Platelets is the most widely ordered haematology test in the Quest network — quantifying and characterising all three major formed blood elements from a single EDTA whole blood specimen using automated multi-parameter impedance and fluorescence flow cytometry. The panel delivers the WBC total with 5-part absolute and percentage differential, the complete erythrocyte profile with haemoglobin, haematocrit, and all morphological indices, and the platelet count with MPV — the full haematological picture that underpins clinical decision-making across infection, anaemia, haemostasis, haematoproliferative disease, and medication safety monitoring in a single draw. The CBC occupies a unique position in laboratory medicine as simultaneously the highest-volume single test and one of the highest-yield clinical investigations — a well-executed CBC with differential provides diagnostic direction across a wider range of conditions than almost any other single test at its cost. Acute bacterial infection presents with neutrophilia and left shift; viral infection with reactive lymphocytosis; iron deficiency with microcytosis, elevated RDW, and thrombocytosis; B12 or folate deficiency with macrocytosis and hypersegmented neutrophils; leukaemia with abnormal cell populations triggering manual differential flag; and thrombocytopenic purpura with severe isolated thrombocytopenia. None of these patterns requires a separate order — they are all revealed by the same CBC specimen interpreted alongside the clinical presentation. In the Quest clinical context — a full-service national laboratory with extensive result interpretation support — the CBC with differential and platelets is the haematological foundation of every comprehensive evaluation from annual wellness checks to complex oncological workup.
What does the CBC with Differential test measure?
Leucocytes are produced from haematopoietic stem cells in the bone marrow under the control of colony-stimulating factors — G-CSF (neutrophils), M-CSF (monocytes), IL-5 (eosinophils), thrombopoietin — and released into the peripheral circulation where they form two pools: a freely circulating pool measured by venepuncture and a marginated pool adherent to the vascular endothelium, releasable by catecholamine-driven demargination. Automated 5-part differential analysis uses a combination of cell volume measurement by electrical impedance, light scatter at multiple angles (forward, side, depolarised), and fluorescence from nucleic acid staining to classify leucocytes into neutrophil, lymphocyte, monocyte, eosinophil, and basophil populations. Absolute counts — derived by multiplying the percentage differential by the total WBC — are the more clinically precise values for assessing individual leucocyte populations independently of other population shifts.
The WBC total count and 5-part differential is the primary haematological screen for infectious, inflammatory, allergic, immunological, and haematoproliferative conditions. Absolute neutrophil count (ANC) is the critical safety threshold for infection risk in immunocompromised patients. Absolute lymphocyte count (ALC) is the primary immune competence marker in HIV monitoring and post-chemotherapy recovery assessment. The neutrophil-to-lymphocyte ratio (NLR) is a validated prognostic marker across cardiovascular disease, malignancy, sepsis, and inflammatory conditions — rising NLR indicates a shift toward innate immune predominance consistent with systemic inflammatory activation. Eosinophilia directs investigation toward atopic disease, parasitic infection, drug reactions, or — at higher counts — eosinophilic granulomatosis with polyangiitis and eosinophilic leukaemia.
Erythrocytes are the most abundant blood cell type — approximately 5 million per µL — produced by erythropoiesis in the bone marrow from erythroid progenitor cells under EPO regulation, maturing over 7 days through stages characterised by progressive haemoglobin accumulation and nuclear extrusion. The final nucleate reticulocyte stage lasts 24–48 hours in peripheral blood before fully maturing into a biconcave anucleate erythrocyte. The erythrocyte indices derive from automated light scatter and impedance measurements: MCV provides the mean single-cell volume in femtolitres; MCH the mean per-cell haemoglobin mass in picograms; MCHC the mean intracellular haemoglobin concentration in g/dL; and RDW-CV (coefficient of variation of RBC volume distribution) the percentage spread around the mean MCV — a sensitive indicator of size heterogeneity (anisocytosis) that rises early in developing deficiency states.
The erythrocyte parameters provide the primary anaemia characterisation data. Haemoglobin below the sex-specific lower limit confirms anaemia with clinical urgency proportional to severity. The MCV-based morphological classification is the pivotal first branch in the anaemia diagnostic pathway: microcytic (MCV <79 fL) directs to iron deficiency, thalassaemia trait, and sideroblastic anaemia workup; normocytic (MCV 79–97 fL) to haemolysis, anaemia of chronic disease, and bone marrow failure; macrocytic (MCV >97 fL) to B12 and folate deficiency, hypothyroidism, liver disease, and medication toxicity. RDW elevation above 15.0% — preceding MCV change by weeks in evolving iron, B12, and folate deficiency — is the earliest automated haematological signal of nutritional depletion and provides additional discrimination in the normocytic anaemia differential between single-cause anaemia (normal RDW) and mixed or early deficiency (elevated RDW).
Platelets are the smallest formed blood element — anucleate cytoplasmic disc-shaped fragments shed by bone marrow megakaryocytes at a rate of approximately 1 × 10¹¹ per day, regulated by thrombopoietin acting on c-Mpl receptors. They circulate for 8–12 days before splenic sequestration and reticuloendothelial clearance. Functionally, platelets exist in a resting state in circulation and become activated through contact with exposed subendothelial matrix (collagen, von Willebrand factor) and soluble agonists (ADP, thromboxane A2, thrombin), transitioning to a sticky, procoagulant form that adheres, activates further, and aggregates to form the primary haemostatic plug. Mean platelet volume reflects the average platelet size — young platelets released during high-demand states are larger and more metabolically active, with greater granule density and higher thrombogenic capacity.
Platelet count assessment is foundational to haemostasis and bleeding risk management. Thrombocytopenia (<140–150 ×10³/µL) stratified by severity: mild (100–150 ×10³/µL) — monitor and investigate cause; moderate (50–100 ×10³/µL) — significant surgical bleeding risk; severe (<50 ×10³/µL) — spontaneous mucocutaneous bleeding risk; critical (<20 ×10³/µL) — spontaneous intracranial and visceral haemorrhage risk requiring urgent management. Causes span immune thrombocytopenic purpura (ITP), drug-induced thrombocytopenia, bone marrow failure, microangiopathic haemolytic anaemia (TTP, HUS), hypersplenism, and disseminated intravascular coagulation (DIC). Thrombocytosis (>400 ×10³/µL) — reactive causes (iron deficiency, infection, post-splenectomy, inflammation) are far more common than clonal essential thrombocythaemia; platelet counts persistently above 600–800 ×10³/µL require JAK2 mutation testing and haematology referral. MPV elevation is associated with platelet hyperactivity and increased arterial thrombotic risk — a cardiovascular risk marker gaining clinical traction alongside hs-CRP and NLR.
Why is the CBC with Differential test ordered?
- General health screening and annual wellness assessment — CBC as the foundational haematological screen establishing baseline blood cell counts and indices, detecting early haematological abnormalities before symptoms emerge, and providing longitudinal trend data for individualised health monitoring
- Infection evaluation and severity stratification — WBC with differential characterising the pattern of immune response: neutrophilia with left shift indicating acute bacterial infection; lymphocytosis indicating viral aetiology; marked leucocytosis or leucoerythroblastic picture indicating severe systemic infection requiring urgent assessment
- Anaemia investigation — haemoglobin, MCV, MCH, MCHC, and RDW for morphological classification establishing the first branch of the anaemia diagnostic pathway and directing targeted second-tier investigation (iron panel, B12, folate, haemoglobin electrophoresis, reticulocyte count)
- Pre-medication safety baseline and monitoring — CBC before initiating chemotherapy, methotrexate, azathioprine, clozapine, carbimazole, hydroxyurea, mycophenolate, or any agent with haematological adverse effect profiles; serial monitoring to detect cytopenias requiring dose modification or treatment interruption
- Bleeding tendency and coagulopathy evaluation — platelet count in patients presenting with easy bruising, petechiae, mucocutaneous bleeding, or prolonged bleeding after minor injury; severity of thrombocytopenia determines the urgency of haematological investigation and intervention
- Haematoproliferative disease investigation and monitoring — CBC with differential for initial screening when leukaemia, lymphoma, myeloma, or myeloproliferative neoplasm is suspected clinically; serial CBCs for disease monitoring and treatment response assessment in established haematological malignancy
- Chronic disease haematological monitoring — serial CBC in HIV (immune reconstitution and ART toxicity), CKD (anaemia of CKD severity and EPO response), inflammatory bowel disease (anaemia and leucocyte pattern monitoring), and autoimmune conditions (cytopenia screening and drug toxicity surveillance)
- Post-operative and post-procedure haematological assessment — haemoglobin and platelet count following surgery, invasive procedures, or significant blood loss to quantify haemorrhagic consequences and guide transfusion and haemostatic management decisions
Sample collection and turnaround
Sample type
Blood (whole blood)
Fasting required
No — fasting does not affect haematological parameters and is not required for the CBC. Pre-analytical specimen handling technique is the dominant determinant of result quality for this test.
Collection method
Venipuncture at patient service centre
Turnaround
1–2 business days
Collection notes
Collect into an EDTA anticoagulant tube (lavender / purple top, K2-EDTA preferred by most automated haematology platforms). Observe the following immediately after collection — these steps are non-negotiable for CBC accuracy: (1) fill the tube to within 10% of the indicated fill line — underfilling distorts the EDTA:blood ratio; (2) gently invert 8–10 times immediately after capping — thorough mixing is essential to prevent microclot formation and EDTA-dependent pseudothrombocytopenia; (3) do not shake vigorously — shaking causes haemolysis; (4) do not refrigerate — cold causes erythrocyte cold agglutination and platelet clumping; (5) transport to Quest Diagnostics at ambient temperature (18–25°C) within the maximum acceptable transit window. Document on the Quest requisition form: (1) current medications affecting haematological parameters — chemotherapy, methotrexate, immunosuppressants, anticoagulants, clozapine, carbimazole, hydroxyurea; (2) acute symptoms or illness on the collection day — fever, infection, acute physiological stress; (3) recent vigorous exercise within 24 hours; (4) corticosteroid type, dose, and duration; (5) pregnancy status and gestational age; (6) known haemoglobinopathy — sickle cell trait or disease, thalassaemia. # REVIEWER FLAG: Confirm Quest Diagnostics EDTA tube type preference (K2-EDTA vs K3-EDTA), required fill volume tolerances, maximum transit time at room temperature, and whether Quest applies a platelet film reflex protocol for automated platelet clumping flags.
Specimen requirements
Minimum 3.0 mL EDTA whole blood (standard 4.0 mL draw into a fully-filled EDTA tube). Analyse or deliver within 6–8 hours at ambient temperature (18–25°C) for optimal WBC morphology, differential accuracy, and MPV reliability. Do not freeze. Do not refrigerate. # REVIEWER FLAG: Confirm Quest Diagnostics minimum volume requirement and maximum acceptable transit time — Quest's specific analyser platform determines the stability window for reliable 5-part differential and extended parameters.
What can affect CBC with Differential results?
The EDTA tube must be filled to within 10% of the indicated fill volume — underfilling increases the EDTA:blood ratio, causing platelet shrinkage and erythrocyte crenation that produces artefactual reduction in MCV and platelet volume estimates. Thorough immediate mixing — 8–10 gentle inversions — is the single most critical collection technique requirement; inadequate mixing allows fibrin microclots to form that trap platelets, causing pseudothrombocytopenia. EDTA-dependent platelet clumping affects approximately 0.1–0.2% of patients independently of tube fill — producing severely suppressed platelet counts in non-thrombocytopenic individuals. Quest Diagnostics' analyser flagging for platelet clumping should trigger film confirmation before thrombocytopenia investigation is initiated.
WBC morphological integrity for differential accuracy begins to degrade within 4–6 hours at room temperature — vacuolation and nuclear changes in neutrophils and monocytes can produce artefactual cellular abnormalities that mimic toxic granulation or dysplastic changes on automated analysis. Platelet MPV progressively increases artefactually as platelets swell in vitro over time. Refrigeration causes cold agglutination of erythrocytes, elevating MCV and producing spurious MCV-MCH-MCHC discordance. Quest specifies a maximum room-temperature transit time that must be respected for reliable differential and extended parameter reporting — confirm this window with Quest Diagnostics.
Systemic glucocorticoids produce a reproducible and clinically recognisable CBC differential pattern: neutrophilia from demargination and reduced apoptosis; lymphopenia from lymphoid redistribution; eosinopenia from sequestration; monocytopenia — collectively the 'steroid differential'. This pattern should not be attributed to bacterial infection or primary leucocyte disease when glucocorticoid use is documented. The steroid differential may persist for 24–48 hours after a single corticosteroid dose and longer with ongoing therapy. Concurrent corticosteroid dose and duration should always be documented on Quest requisition forms when interpreting the CBC differential.
Acute physiological stressors — vigorous exercise, emotional distress, pain, anxiety, or sympathoadrenal activation immediately before collection — mobilise the marginated neutrophil pool from vascular endothelium into the circulating pool through catecholamine-mediated beta-2 receptor activation, transiently elevating WBC and neutrophil count by 50–100% without underlying pathology. Collection during or immediately following significant physical activity, a stressful procedure, or an anxiety-provoking event should be documented as a potential source of acute physiological leucocytosis distinct from infection-pattern neutrophilia.
In patients with extreme leucocytosis (WBC >100 ×10³/µL) — as in CML blast crisis or acute leukaemia — or extreme thrombocytosis (>1000 ×10³/µL), in vitro cellular lysis and potassium release during specimen processing can produce pseudohyperkalaemia on serum chemistry panels ordered concurrently with the CBC. Additionally, blast cells and abnormal leucocyte populations may be misclassified by automated differential algorithms, producing a differential report that underrepresents the true blast percentage — manual microscopic film review is mandatory when blasts or abnormal populations are suspected or flagged.
Haemoglobin S (sickle cell trait or disease) produces characteristic CBC changes: normal or mildly reduced haemoglobin in sickle cell trait; moderate anaemia with elevated RDW, variable MCV, and elevated MCHC in sickle cell disease; and the pathognomonic sickle cell forms and target cells on peripheral film. Sickle cell disease erythrocytes have a shortened lifespan (20–120 days depending on disease severity), producing a haemolytic reticulocytosis that may cause MCV and MCH to appear relatively normal despite ongoing haemolysis. Clinicians should document known haemoglobinopathy on the Quest requisition form for appropriate result interpretation and potential film review flagging.
What do CBC with Differential results mean?
Elevated
Leucocytosis with differential pattern determines clinical direction: neutrophilia (ANC >7.7 ×10³/µL) — acute bacterial infection, physiological stress response, corticosteroid effect, or myeloproliferative disease; lymphocytosis (ALC >5.2 ×10³/µL) with reactive lymphocyte morphology indicates viral infection (EBV, CMV, primary HIV); persistent lymphocytosis with monotonous small lymphocytes in a patient over 50 raises concern for CLL — flow cytometry lymphocyte immunophenotyping is the next step; eosinophilia (>0.7 ×10³/µL) triggers evaluation for atopic disease, drug reaction, parasitic infection, or hypereosinophilic syndrome based on magnitude. Erythrocytosis (Hgb above sex-specific upper limit) — exclude secondary causes (smoking, altitude, sleep apnoea, EPO-producing tumour, testosterone therapy) before primary polycythaemia vera evaluation with JAK2 V617F mutation testing. Thrombocytosis above 400 ×10³/µL — reactive causes far outnumber clonal; assess for iron deficiency (concurrent ferritin), active infection, inflammation, and post-splenectomy; persistent thrombocytosis above 600–800 ×10³/µL warrants JAK2 testing and haematology referral.
Normal
A fully normal CBC with differential in a symptomatic patient provides robust reassurance that the most common haematological contributors to the presenting complaint — anaemia, infection-pattern leucocytosis, thrombocytopenia — are absent at the time of testing. A normal CBC does not exclude early haematological malignancy with normal peripheral blood (early MDS, Hodgkin lymphoma), compensated haemolytic anaemia with reticulocytosis maintaining normal haemoglobin, platelet functional disorders (von Willebrand disease, platelet function defects) producing bleeding despite normal platelet count, or pre-anaemic nutritional deficiency before morphological changes develop. When the clinical probability for a haematological disorder remains high despite a normal CBC, targeted further investigation — reticulocyte count, peripheral blood film, flow cytometry, bone marrow assessment — remains appropriate.
Reduced
Anaemia (Hgb below sex-specific lower limit) — MCV-based classification drives next steps: microcytic (iron panel, haemoglobin electrophoresis for thalassaemia); normocytic (reticulocyte count, B12, folate, renal function, inflammatory markers); macrocytic (B12, folate — both mandatory simultaneously before supplementation). Neutropenia (ANC <1.8 ×10³/µL): mild (1.0–1.8) — investigate medications, viral illness, autoimmune neutropenia; severe (<0.5 ×10³/µL) — urgent haematology referral, infection risk assessment, and isolation precautions if symptomatic. Thrombocytopenia (<140 ×10³/µL): exclude EDTA pseudothrombocytopenia by film and alternative tube recount; if confirmed, severity guides urgency — below 20 ×10³/µL is haematological emergency. Pancytopenia (WBC, Hgb, and platelets all reduced simultaneously) indicates bone marrow failure — aplastic anaemia, haematological malignancy infiltration, or myelodysplastic syndrome — requiring urgent haematology referral and bone marrow biopsy consideration.
Note: The automated CBC differential is a quantitative classifier — it cannot replace manual microscopic blood film examination for qualitative cell morphology assessment. When the Quest analyser flags abnormal populations (blasts, atypical lymphocytes, schistocytes, NRBCs, hypersegmented neutrophils, giant platelets), a pathologist-reviewed manual differential is mandatory before clinical decisions are based on the differential result. MCHC above 36 g/dL is the automated flag for in vitro haemolysis — any CBC with MCHC above this threshold requires haemolysis assessment before the erythrocyte parameters are used clinically. Pseudothrombocytopenia from EDTA-dependent platelet clumping is the most common cause of an isolated severely low platelet count in a non-bleeding well patient — always confirm with a blood film before initiating a thrombocytopenia investigation. The 5-part differential cannot reliably distinguish reactive lymphocytosis from early lymphoid malignancy — flow cytometry immunophenotyping is required for this clinical discrimination.
Frequently asked questions
The CBC with differential and platelets measures three blood cell lineages from a single EDTA whole blood specimen: white blood cells — total count plus 5-part differential (neutrophils, lymphocytes, monocytes, eosinophils, basophils) with both percentage and absolute counts; red blood cells — count, haemoglobin, haematocrit, and morphological indices (MCV, MCH, MCHC, RDW); and platelets with mean platelet volume. Together these identify anaemia and classify its type, characterise immune and inflammatory responses, screen for haematoproliferative disease, and assess haemostatic platelet reserve.
How to access the CBC with Differential test
The CBC with Differential reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: May 24, 2026
Last reviewed: May 24, 2026
- Bain BJ. Blood cells: a practical guide. 5th ed. Wiley-Blackwell. (2015)
- Lippi G, et al. Preanalytical variability in laboratory testing: current issues and future perspectives. Ann Transl Med. (2017)
- Bhat T, et al. Neutrophil to lymphocyte ratio and cardiovascular diseases: a review. Expert Rev Cardiovasc Ther. (2013)
- Slavka G, et al. Mean platelet volume may represent a predictive parameter for overall vascular mortality and ischemic heart disease. Arterioscler Thromb Vasc Biol. (2011)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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