Anemia Workup
Classify by MCV first -microcytic, normocytic, or macrocytic. Then use iron studies, reticulocyte count, and peripheral smear to narrow the diagnosis. Iron deficiency is the most common cause worldwide.
🔍 MCV-Based Classification
| MCV | Category | Differential | Key Labs |
|---|---|---|---|
| < 80 | Microcytic | Iron deficiency (#1 -GI blood loss in men/postmenopausal women until proven otherwise), thalassemia (Mentzer index: MCV/RBC < 13 → thalassemia, > 13 → iron def), anemia of chronic disease (some), sideroblastic, lead poisoning | Iron studies, ferritin, TIBC, reticulocyte count, Hgb electrophoresis if thalassemia suspected |
| 80–100 | Normocytic | Anemia of chronic disease (#1), acute blood loss, CKD (↓ EPO), mixed deficiency (iron + B12), hemolysis (check hemolysis labs), bone marrow failure (aplastic, MDS, infiltration) | Reticulocyte count (↑ = destruction/loss, ↓ = underproduction), BMP (CKD), hemolysis labs (LDH, haptoglobin, indirect bili, smear) |
| > 100 | Macrocytic | B12 deficiency (neurologic symptoms -subacute combined degeneration; ask about nitrous oxide abuse, where the B12 level is usually normal and the CBC is usually normal too), folate deficiency, alcohol/liver disease, hypothyroidism, MDS, medications (methotrexate, hydroxyurea, azathioprine, AZT), reticulocytosis (reticulocytes are large → MCV ↑) | B12, folate, reticulocyte count, TSH, peripheral smear (hypersegmented neutrophils → megaloblastic), methylmalonic acid (↑ in B12 def, normal in folate def) |
New iron deficiency anemia in a man or postmenopausal woman = GI malignancy until proven otherwise. These patients need colonoscopy + upper endoscopy. Do NOT just give iron and move on.
Reticulocyte Count -Production vs Destruction
- Reticulocyte count > 2% (or reticulocyte index > 2): bone marrow is responding → the problem is blood loss or destruction (hemolysis)
- Reticulocyte count < 2%: bone marrow is NOT responding → underproduction (iron/B12/folate deficiency, CKD, bone marrow failure, anemia of chronic disease)
- Reticulocyte Production Index (RPI): (retic % × Hgb/15) / maturation factor. RPI > 2 = appropriate response.
🧪 Iron Studies Interpretation
| Lab | Iron Deficiency | Anemia of Chronic Disease | Both (Mixed) |
|---|---|---|---|
| Serum iron | ↓ | ↓ | ↓ |
| Ferritin | ↓↓ (< 30) | ↑ or normal (acute phase reactant) | Low-normal (30–100) |
| TIBC | ↑ (body wants more iron) | ↓ (body not trying to absorb more) | Variable |
| Transferrin saturation (TSAT) | ↓ (< 20%) | ↓ (15–20%) | ↓ |
Ferritin is an acute phase reactant -it rises with inflammation, infection, liver disease, and malignancy. A "normal" ferritin of 50–100 in an inflamed hospitalized patient may actually represent iron deficiency. Ferritin < 30 is diagnostic of iron deficiency. Ferritin < 100 in the setting of inflammation is suggestive.
What is TSAT (Transferrin Saturation)? The percentage of transferrin (the iron-carrying transport protein) that is currently bound to iron. Formula: TSAT = (serum iron ÷ TIBC) × 100. Normal range: 20-50%. Iron deficiency: < 20%. Iron overload: > 45-50% (consider hemochromatosis or transfusion overload).
Why it complements ferritin: TSAT reflects the iron actually available right now for erythropoiesis (the "circulating" iron pool), while ferritin reflects storage iron. TSAT is less affected by inflammation, so it's the more reliable marker when ferritin is unreliable -hospitalized inflamed patients, HF, CKD, chronic infection, malignancy. A patient with ferritin 80 + TSAT 12% is iron-deficient even though the ferritin looks "OK."
Pre-test pearls: hold oral iron for 24-48 h before drawing labs (recent iron pill falsely elevates TSAT). TSAT has a diurnal rhythm -higher in the morning -so a fasting AM draw is most reliable when the result is borderline.
Why it complements ferritin: TSAT reflects the iron actually available right now for erythropoiesis (the "circulating" iron pool), while ferritin reflects storage iron. TSAT is less affected by inflammation, so it's the more reliable marker when ferritin is unreliable -hospitalized inflamed patients, HF, CKD, chronic infection, malignancy. A patient with ferritin 80 + TSAT 12% is iron-deficient even though the ferritin looks "OK."
Pre-test pearls: hold oral iron for 24-48 h before drawing labs (recent iron pill falsely elevates TSAT). TSAT has a diurnal rhythm -higher in the morning -so a fasting AM draw is most reliable when the result is borderline.
Iron Replacement
| Route | Agent | Notes |
|---|---|---|
| Oral | Ferrous sulfate 325 mg PO daily–TID | Take on empty stomach with vitamin C (enhances absorption). GI side effects are dose-limiting. Every-other-day dosing may improve absorption and tolerability. Takes 3–6 months to replete stores. |
| IV (preferred if) | Iron sucrose (Venofer) 200 mg IV × 5 doses or ferric carboxymaltose (Injectafer) 750 mg IV × 2 | Preferred if: oral intolerant, CKD/dialysis, IBD, ongoing blood loss exceeding oral repletion, Hgb < 7 with symptoms, pre-surgery. Injectafer: can give 750 mg in one sitting (fewer visits). |
🧪 Workup
Workup
One question organizes the entire workup: is the marrow failing to make red cells, or are they being lost or destroyed? The reticulocyte count answers it, which is why it is ordered with the very first CBC and not after it. A low or inappropriately normal retic means underproduction, so the workup turns to iron, B12, folate, kidney and marrow. A high retic means the marrow is responding correctly to bleeding or hemolysis, so the workup turns to finding the blood or the destruction. Ordering the MCV without the retic tells you what the cells look like but not what is happening.
| Test | What You Are Looking For | Why It Matters |
|---|---|---|
| CBC with reticulocyte count FIRST | The MCV sorts the cause by size (microcytic, normocytic, macrocytic) and the retic sorts it by mechanism. | Correct the retic for the degree of anemia, since a raw percentage of a small red cell mass overstates the response. An untreated iron deficiency with a "normal" retic is a marrow that is failing to respond, not one that is coping. |
| Peripheral smear DO NOT SKIP | Schistocytes, spherocytes, bite cells, target cells, hypersegmented neutrophils, teardrops, blasts, rouleaux. | ⚠ Schistocytes change the night. They point to a microangiopathic process, and TTP is a hematologic emergency where the delay to plasma exchange is what kills. Hypersegmented neutrophils flag B12 or folate deficiency before the MCV rises, and blasts mean the diagnosis is leukemia, not anemia. No analyzer index substitutes for looking. |
| Iron studies | Ferritin < 30 ng/mL is diagnostic of iron deficiency (sensitivity 92%, specificity 98%). Transferrin saturation < 20% supports it. | ⚠ Ferritin is an acute phase reactant, so inflammation can lift it into the normal range in a genuinely iron-deficient patient. In chronic inflammation, raise the cutoff toward 100 ng/mL and lean on transferrin saturation instead. This is the commonest way iron deficiency gets missed on a normal-looking panel. |
| B12 and folate | B12 below roughly 200 pg/mL. Check methylmalonic acid and homocysteine when the level is borderline, and always if nitrous oxide is in the history, where B12 is only 22% sensitive but homocysteine and MMA are over 90% (why). | The borderline zone is common and the metabolites settle it, because MMA rises in true B12 deficiency and stays normal in folate deficiency. It matters because giving folate alone to a B12-deficient patient corrects the anemia while the neurologic damage progresses, and that damage can be permanent. |
| Hemolysis panel | LDH high, haptoglobin low, indirect bilirubin high, retic high. All four move together. | Haptoglobin is the most useful single member: it is consumed by free hemoglobin, so a low haptoglobin with a high LDH is strong evidence of hemolysis. Interpret them as a set, since LDH alone rises in almost anything. |
| Direct antiglobulin test (Coombs) | Antibody or complement on the red cell surface. | Splits confirmed hemolysis into immune and non-immune, which is the branch point for treatment. Positive points to autoimmune hemolysis and steroids; negative redirects you to microangiopathy, a mechanical valve, G6PD or a membrane defect. |
| Renal function and EPO | Creatinine, eGFR, and an erythropoietin level where the picture is unclear. | Anemia of chronic kidney disease is an erythropoietin deficiency, and it is common enough to explain many normocytic anemias without further hunting. Correct iron first, since an ESA will not work in an iron-deficient patient. |
| Bone marrow biopsy | Reserved for when the non-invasive workup does not explain it. | Indicated for pancytopenia, circulating blasts, suspected myelodysplasia or infiltration. It is the answer to an unexplained hypoproliferative anemia, not a first-line test. |
⚠ Finding iron deficiency is only half the diagnosis. You then have to find where the iron went. In an adult man or a postmenopausal woman, iron deficiency is a gastrointestinal malignancy until proven otherwise and warrants endoscopic evaluation of both upper and lower tracts. Prescribing iron and discharging is the error: the count comes up, everyone is reassured, and the tumor goes on bleeding.
🚨 Management
Management
- Iron deficiency: FIND SOURCE + IV iron (ferric carboxymaltose 750mg×2) or PO. Transfuse: Hgb<7 in most patients TRICC, Hébert 1999
- B12: IM cyanocobalamin 1000mcg daily×7→weekly×4→monthly
- Hemolytic: autoimmune→steroids±rituximab. TTP→PLEX. Mechanical→treat cause.
- CKD: iron first, then ESA (Hgb 10-11)
- Transfuse: Hgb<7 most, <8 ACS TRISS, Holst 2014 TITRe2, Murphy 2015
📋 On Rounds
How do you distinguish iron deficiency from anemia of chronic disease when both have low serum iron?
Ferritin and TIBC. In iron deficiency, the body is truly depleted of iron → ferritin is low (< 30) and TIBC is high (the body upregulates transferrin to grab any available iron). In anemia of chronic disease, iron is trapped in macrophages by hepcidin (the inflammatory "iron warden") → ferritin is normal or high (iron is stored, just not released) and TIBC is low (no need to absorb more -there's plenty, just sequestered).
A patient has a ferritin of 80 but you still suspect iron deficiency. How is that possible?
Ferritin is an acute phase reactant -it rises with inflammation, infection, liver disease, and malignancy regardless of iron stores. A 'normal' ferritin of 80 in a patient with active inflammation (elevated CRP) may actually represent iron deficiency. In the setting of chronic disease or inflammation, a ferritin < 100 with transferrin saturation < 20% suggests functional iron deficiency even though ferritin appears 'normal'.
A patient has pancytopenia. What's your differential and what do you order?
Pancytopenia = all three lines down (anemia + leukopenia + thrombocytopenia). Differential by mechanism: Decreased production (bone marrow failure): aplastic anemia, MDS, leukemia/lymphoma, myelofibrosis, B12/folate deficiency (megaloblastic), drug-induced (methotrexate, chemotherapy), infections (HIV, hepatitis, parvovirus, TB). Increased destruction: hypersplenism (cirrhosis), DIC, TTP/HUS, autoimmune (SLE). Workup: CBC with peripheral smear (blasts → leukemia; schistocytes → TTP/DIC; hypersegmented neutrophils → B12/folate; teardrop cells → myelofibrosis). Reticulocyte count (↓ = production problem, ↑ = destruction/loss). B12, folate, LDH, haptoglobin, Coombs. HIV, hepatitis serologies. Fibrinogen, D-dimer, PT/PTT if DIC suspected. ADAMTS13 if TTP suspected. SPEP/free light chains if myeloma on smear (rouleaux). Bone marrow biopsy if etiology unclear after initial labs, or if blasts / leukoerythroblastic picture on smear.
How do you approach anemia when the MCV is normal (normocytic)?
Normocytic anemia (MCV 80-100): the broadest differential. Step 1: Reticulocyte count. High retic (> 2%): the marrow is responding → blood LOSS or blood DESTRUCTION. Check: LDH, haptoglobin, Coombs test. If hemolysis markers positive → hemolytic anemia (autoimmune, TTP, DIC, G6PD, sickle cell, PNH). If hemolysis markers negative → acute blood loss (look for the source). Low retic (< 2%): marrow is NOT responding → production problem.
Clinical Examples
📋 Case 1, Microcytic Anemia from Menorrhagia
Patient: 34-year-old premenopausal woman with heavy menstrual periods presents with fatigue and pica (ice craving).
Key findings: Hgb 7.6, MCV 65, ferritin 4, TIBC 520, TfSat 4%, reticulocyte index 0.5%. Smear: microcytic hypochromic cells, pencil cells.
Management:
- IV iron (ferric carboxymaltose 750 mg x 2) due to severity
- Gynecology referral for menorrhagia management
- GI workup deferred given clear gynecologic source and young age
Teaching point: Pica (pagophagia) is specific for iron deficiency. In premenopausal women, menorrhagia is the most common cause. IRON-MIDE, 2020
📋 Case 2, Normocytic Anemia with Hemolysis
Patient: 42-year-old woman with SLE presents with jaundice, dark urine, and worsening fatigue. Splenomegaly on exam.
Key findings: Hgb 6.2, MCV 102 (elevated from reticulocytosis), reticulocyte count 12%, LDH 680, haptoglobin undetectable, indirect bilirubin 4.8, DAT positive (IgG + C3).
Management:
- Warm AIHA secondary to SLE: prednisone 1 mg/kg/day
- Transfuse if hemodynamically unstable
- If steroid-refractory: rituximab or splenectomy
Teaching point: Warm AIHA (IgG-mediated) is associated with SLE, CLL, and drugs. MCV may be falsely elevated because reticulocytes are larger than mature RBCs. Haptoglobin is the most sensitive hemolysis marker.
📋 Case 3, Macrocytic Anemia from B12 Deficiency
Patient: 68-year-old vegan woman presents with fatigue, glossitis, and bilateral lower extremity paresthesias. Decreased vibration sense, positive Romberg.
Key findings: Hgb 9.2, MCV 124, reticulocyte index 0.6. B12 less than 100. Methylmalonic acid 2400 (markedly elevated). Smear: macro-ovalocytes, hypersegmented neutrophils.
Management:
- IM cyanocobalamin 1000 mcg daily x 7d, then weekly x 4, then monthly lifelong
- Monitor K+ and phosphate (drop during rapid erythropoiesis)
- Neurology follow-up for subacute combined degeneration
Teaching point: B12 deficiency causes subacute combined degeneration (posterior columns + lateral corticospinal tracts). Methylmalonic acid differentiates B12 from folate deficiency. Neurologic damage may be irreversible if treatment is delayed.
📣 Sample Presentation
One-Liner
"Mrs. Rivera is a 48-year-old premenopausal woman with fatigue and Hgb 8.2, MCV 72. Iron studies: ferritin 8, TIBC 450, transferrin sat 6%. Consistent with iron deficiency anemia."
Key Points to Cover on Rounds
Microcytic anemia -iron deficiency confirmed (ferritin 8, Tsat 6%). Source: heavy menstrual bleeding (menorrhagia × 2 years, soaking through pads). No GI symptoms, no melena. Given her age and clear gynecologic source, GI workup deferred. Treatment: IV iron infusion (ferric carboxymaltose 750 mg × 2 doses, 1 week apart) -chose IV over PO due to severity and faster repletion. Gynecology referral for menorrhagia management. Plan: recheck CBC + ferritin in 6-8 weeks, expect Hgb rise 1 g/dL per week of iron.
💊 Medications
Key Medications -Anemia
Treatment depends on the cause. Iron deficiency = replace iron + find source. B12 deficiency = replace B12. Anemia of CKD = iron first, then ESA. Hemolytic = treat underlying cause.
| Drug | Dose | Route | Indication |
|---|---|---|---|
| Ferrous sulfate | 325 mg PO daily–TID | PO | Iron deficiency anemia (mild-moderate). Take on empty stomach with vitamin C. Every-other-day dosing may improve absorption. |
| Iron sucrose (Venofer) | 200 mg IV x 5 doses | IV | IV iron preferred if: oral intolerant, CKD/dialysis, IBD, Hgb < 7, ongoing blood loss, pre-surgery. |
| Ferric carboxymaltose (Injectafer) | 750 mg IV x 2 (1 week apart) | IV | Convenient IV iron -can give 750 mg in one sitting (fewer visits). |
| Cyanocobalamin (B12) | 1000 mcg IM daily x 7d → weekly x 4 → monthly | IM | B12 deficiency. High-dose oral (1000–2000 mcg/day) may be adequate if no absorption issues. |
| Folic acid | 1 mg PO daily | PO | Folate deficiency. Always check B12 first -folate alone can mask B12 deficiency (corrects anemia but NOT neuro damage). |
| Epoetin alfa (Procrit) | 50–300 units/kg 3x/week | IV/SQ | Anemia of CKD (after iron repletion). Target Hgb 10–11.5. Never > 13. |
⚡ Summary
Summary
MCV Approach
Microcytic (< 80): iron deficiency, thalassemia, chronic disease, sideroblastic. Normocytic (80-100): chronic disease, CKD, hemolysis, bleeding. Macrocytic (> 100): B12/folate, MDS, liver disease, hypothyroid, drugs.
Iron Deficiency
Ferritin < 30 (or < 100 in CKD/inflammation), low Tsat. Find the source: GI (colonoscopy), menstrual, malabsorption. IV iron for severe/refractory.
Hemolysis Workup
LDH ↑, haptoglobin ↓, indirect bilirubin ↑, reticulocyte ↑. Coombs test: positive = autoimmune. Negative = mechanical (TTP, DIC, prosthetic valve).
Transfusion Threshold
Hgb < 7 for most patients [TRICC, TRISS]. Hgb < 8 for active cardiac ischemia. Never transfuse to a number -transfuse for symptoms.
B12 Deficiency
Can cause pancytopenia mimicking leukemia. Hypersegmented neutrophils on smear. Neuro symptoms (subacute combined degeneration). Check methylmalonic acid.
Reticulocyte Count
The single most important test. High retic = bone marrow is working (loss or destruction). Low retic = production failure (check iron/B12/folate, consider biopsy).
📄 One Pager
Text version
Hematology · One Pager
Anemia
MCV-based approach: micro (iron), normo (chronic disease, hemolysis, bleeding), macro (B12/folate). Reticulocyte count = production vs destruction. Transfuse Hgb < 7.
MCV Approach
Microcytic (< 80): iron deficiency (#1), thalassemia, chronic disease, sideroblastic. Normocytic (80-100): chronic disease, CKD, hemolysis, acute bleeding. Macrocytic (> 100): B12/folate, MDS, liver disease, hypothyroid, drugs.
Iron Deficiency
Ferritin < 30 (or < 100 in inflammation/CKD), low transferrin sat. FIND THE SOURCE: GI (colonoscopy if > 50 or alarm features), menstrual, malabsorption. IV iron for severe or refractory.
Hemolysis Workup
LDH ↑, haptoglobin ↓, indirect bili ↑, reticulocyte ↑. Coombs positive = autoimmune (warm vs cold). Coombs negative = mechanical (TTP, DIC, valve), PNH, G6PD, sickle cell.
Key Drugs
Ferric carboxymaltose750 mg IV × 2 doses
Cyanocobalamin1000 mcg IM daily × 7d
Folic acid1 mg PO daily
EPO50-300 U/kg 3×/wk (CKD)
Pitfalls
- Transfusing above Hgb 7 threshold without indication
- Not checking reticulocyte count (production vs destruction)
- Missing B12 deficiency (can cause pancytopenia mimicking leukemia)
- Iron infusion without finding the source of loss
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