What is the differential diagnosis for using the different parameters of CBC
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Differential Diagnosis by CBC Parameters
The CBC measures RBCs, WBCs, and platelets — each parameter pointing to a different set of diagnoses. Below is a systematic breakdown.
1. Red Blood Cell (RBC) Parameters
A. Hemoglobin / Hematocrit — Anemia
The first step in evaluating anemia is to classify it by MCV (mean corpuscular volume):
B. MCV — Morphologic Classification of Anemia
| MCV Category | Cutoff | Differential Diagnosis |
|---|---|---|
| Microcytic (↓ MCV) | < 80 fL | Iron deficiency anemia, Thalassemia, Sideroblastic anemia, Lead poisoning, Anemia of chronic disease (also normocytic) |
| Normocytic (normal MCV) | 80–100 fL | Aplastic anemia, Myelofibrosis, Myelophthisic anemia; Hypoendocrine states (hypothyroid, hypoadrenal, hypopituitary), Uremia, Chronic inflammation, Liver disease, Acute blood loss, Hemolysis |
| Macrocytic (↑ MCV) | > 100 fL | Vitamin B12 deficiency, Folate deficiency, Liver disease, Hypothyroidism, Drugs (methotrexate, hydroxyurea), Myelodysplastic syndrome |
For microcytic anemia, serum ferritin < 30 ng/mL confirms iron deficiency. For normocytic anemia, exclude hemolysis (reticulocyte count, LDH, haptoglobin) and renal insufficiency. For macrocytic, check B12/folate levels.
C. RDW — Red Cell Distribution Width
- ↑ RDW + ↓ MCV: Iron deficiency (early), mixed deficiency states
- Normal RDW + ↓ MCV: Thalassemia trait (useful distinguishing feature)
- ↑ RDW + ↑ MCV: B12/folate deficiency, liver disease
- Normal RDW + ↑ MCV: Aplastic anemia, hypothyroidism, reticulocytosis
D. MCHC — Hypochromia / Hyperchromia
- ↓ MCHC (hypochromic): Iron deficiency, thalassemia, sideroblastic anemia
- ↑ MCHC: Hereditary spherocytosis (spherocytes pack hemoglobin more densely)
2. White Blood Cell (WBC) Parameters
A. Leukocytosis — Elevated WBC
The differential depends on which cell line is elevated:
| Cell Type | Condition | Causes |
|---|---|---|
| Neutrophilia | Most common | Bacterial infections, rheumatic/autoimmune disorders, neoplastic disorders, trauma, drugs (corticosteroids, G-CSF), endocrine/metabolic disorders, CML, leukemoid reaction |
| Eosinophilia | Parasitic infections, allergic diseases (asthma, atopic dermatitis), myeloproliferative/neoplastic diseases, cutaneous diseases, GI diseases (IBD), drug reactions | |
| Basophilia | Allergic reactions, CML, myeloid metaplasia, polycythemia vera, ionizing radiation, hypothyroidism, chronic hemolytic anemia, post-splenectomy | |
| Monocytosis | Infections (TB, subacute bacterial endocarditis), neoplastic disorders, GI disorders, sarcoidosis, drug reactions, recovery from marrow suppression | |
| Lymphocytosis | Viral infections (EBV, CMV, hepatitis), lymphocytic leukemia (CLL), pertussis, other infectious diseases, lymphoproliferative neoplasms |
B. Leukopenia — Decreased WBC
| Cell Type | Condition | Causes |
|---|---|---|
| Neutropenia | Overwhelming bacterial infection, viral infections, drug reactions (chemotherapy, carbimazole, clozapine), ionizing radiation, hematopoietic diseases, hypersplenism, anaphylactic shock, autoimmune disease (SLE), aplastic anemia | |
| Eosinopenia | Acute physical stress, acute inflammatory states, Cushing's syndrome, corticosteroid use | |
| Basopenia | Sustained glucocorticoid treatment, acute infection/stress, hyperthyroidism | |
| Monocytopenia | Onset of steroid therapy, hairy cell leukemia | |
| Lymphopenia | Immunodeficiency disorders (HIV/AIDS), adrenocortical hormone excess, chemotherapy, irradiation, impaired intestinal lymphatic drainage, advanced lymphomas/carcinomas, anorexia nervosa |
3. Platelet Parameters
A. Thrombocytopenia — ↓ Platelets
Clinically significant bleeding typically occurs at < 50,000/μL; spontaneous bleeding risk at < 10,000–20,000/μL.
| Mechanism | Causes |
|---|---|
| Decreased Production | Aplastic anemia, congenital disorders, chemotherapy/radiation, vitamin B12 or folate deficiency, drugs, SLE, marrow infiltration (myelophthisis), myelodysplastic syndrome |
| Increased Destruction (Immune) | Immune thrombocytopenia (ITP), drug-induced (heparin → HIT, quinine, sulfonamides), neonatal alloimmune thrombocytopenia, post-transfusion purpura, SLE, antiphospholipid syndrome |
| Increased Destruction (Non-immune) | TTP (thrombotic thrombocytopenic purpura), HUS (hemolytic uremic syndrome), DIC, prosthetic heart valves, preeclampsia/HELLP |
| Sequestration / Dilutional | Hypersplenism (cirrhosis, storage diseases), massive transfusion |
| Factitious | EDTA-dependent platelet aggregation (pseudothrombocytopenia), platelet satellitism — always confirm with peripheral smear |
B. Thrombocytosis — ↑ Platelets
| Type | Causes |
|---|---|
| Reactive (Secondary) | Iron deficiency, acute blood loss, inflammatory disorders (RA, IBD), malignancies, post-splenectomy, infections, trauma — counts rarely exceed 1,000,000/μL |
| Clonal (Primary/Myeloproliferative) | Essential thrombocythemia, polycythemia vera, CML, primary myelofibrosis |
4. Reticulocyte Count (often ordered with CBC)
Though not always reported in a standard CBC, this is the critical next step:
| Reticulocyte Index | Interpretation | Examples |
|---|---|---|
| ↑ (> 2%) | Appropriate bone marrow response / hemolytic or blood-loss anemia | Hemolytic anemia, acute hemorrhage, post-treatment response |
| ↓ (< 2%) | Hypoproliferative / bone marrow failure | Iron deficiency, B12/folate deficiency, aplastic anemia, anemia of chronic disease, renal failure |
Quick Reference Summary
| CBC Parameter | High (↑) | Low (↓) |
|---|---|---|
| Hgb/Hct | Polycythemia vera, dehydration, COPD | Anemia (classify by MCV) |
| MCV | B12/folate deficiency, liver disease, hypothyroid | Iron deficiency, thalassemia, sideroblastic anemia |
| RDW | Mixed deficiencies, early iron/B12 deficiency | Typically normal; low RDW in thalassemia |
| WBC total | Infection, leukemia, steroids, stress | Chemotherapy, aplasia, viral illness, autoimmune |
| Neutrophils | Bacterial infection, steroids, CML | Viral illness, drugs, aplastic anemia, autoimmune |
| Lymphocytes | Viral infection, CLL, pertussis | HIV, steroids, chemo, lymphoma |
| Eosinophils | Parasites, allergy, malignancy | Cushing's, steroids, acute stress |
| Monocytes | TB, SBE, malignancy, sarcoidosis | Hairy cell leukemia, steroids |
| Basophils | CML, allergy, hypothyroid | Stress, hyperthyroidism, steroids |
| Platelets | Iron deficiency, inflammation, post-splenectomy, ET | ITP, TTP, HIT, DIC, aplasia, B12/folate deficiency |
Sources: Textbook of Family Medicine 9e (Speicher); Rosen's Emergency Medicine 9e; Goldman-Cecil Medicine International Edition; Tietz Textbook of Laboratory Medicine 7th Ed.
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