What laboratory tests should be ordered for a patient with low hemoglobin?

Laboratory Workup for Low Hemoglobin

Order a complete blood count with red cell indices (MCV, MCH, MCHC), serum ferritin, transferrin saturation, serum iron, TIBC, reticulocyte count, and peripheral blood smear as your initial laboratory panel. 1, 2, 3

Core Initial Laboratory Tests

Essential First-Line Tests

• Complete blood count with indices – MCV, MCH, and MCHC provide sensitive indication of iron deficiency and classify the anemia pattern (microcytic, normocytic, or macrocytic). 1, 3

• Serum ferritin – This is the most powerful single test for iron deficiency, with <30 µg/L confirming deficiency in the absence of inflammation. 1, 3

• Transferrin saturation (TSAT) – Values <15-16% indicate iron deficiency; calculate from serum iron and TIBC. 2, 3

• Serum iron and total iron-binding capacity (TIBC) – Low serum iron with elevated TIBC is the classic pattern of iron deficiency. 2, 3

• Reticulocyte count – A low or inappropriately normal reticulocyte count indicates inadequate bone marrow response (nutritional deficiency, marrow failure, chronic kidney disease, inflammation), while an elevated count suggests hemolysis or acute blood loss. 2, 4

• Peripheral blood smear – Essential to detect dysplastic cells, blasts, hypochromic/microcytic red cells, target cells (suggesting thalassemia), or other morphological clues to underlying pathology. 2, 3

Additional Tests Based on Clinical Context

Inflammatory and Chronic Disease Markers

• C-reactive protein (CRP) or ESR – Elevated inflammatory markers help interpret ferritin results, as ferritin is an acute-phase reactant that can be falsely elevated by inflammation, infection, or malignancy. 2, 3

• Comprehensive metabolic panel – Evaluates renal function (BUN, creatinine, eGFR) to identify chronic kidney disease as a cause of normocytic anemia due to erythropoietin deficiency. 2

Nutritional Deficiency Screening

• Vitamin B12 and folate levels – Excludes macrocytic anemias due to cobalamin or folate deficiency, particularly important if MCV is elevated or high-normal. 2

• Celiac disease screening – All patients with confirmed iron deficiency anemia should be screened for celiac disease, as malabsorption is an important treatable cause. 1

Hemoglobinopathy Evaluation

• Hemoglobin electrophoresis – Recommended when microcytosis and hypochromia are present in patients of appropriate ethnic background (Mediterranean, African, Southeast Asian descent) to prevent unnecessary gastrointestinal investigation for thalassemia trait. 1

• Red cell distribution width (RDW) – An elevated RDW favors iron deficiency over thalassemia trait. 3

Interpretation Framework for Iron Studies

Iron Deficiency Patterns

• Ferritin <30 µg/L without inflammation → confirms iron deficiency. 2, 3

• Ferritin 30-100 µg/L with elevated CRP → possible iron deficiency masked by inflammation. 2, 3

• TSAT <15-16% → indicates functional iron deficiency. 2, 3

Anemia of Chronic Disease

• Ferritin >100 µg/L with TSAT <20% → suggests anemia of chronic disease rather than iron deficiency. 2

• Low serum iron with normal or low TIBC → distinguishes chronic disease from iron deficiency (which shows elevated TIBC). 2, 3

Critical Pitfalls to Avoid

• Do not rely on MCV alone – Normal-range MCV can mask early iron deficiency; low MCH and MCHC are more reliable early indicators. 3

• Do not start empiric iron therapy before obtaining iron studies – This obscures diagnostic results and delays identification of serious underlying causes. 3

• Do not attribute anemia solely to menstruation without confirming iron deficiency – Ferritin <30 µg/L without clear menstrual loss warrants gastrointestinal evaluation for occult bleeding. 3

• Always interpret ferritin alongside inflammatory markers – Inflammation can falsely elevate ferritin, masking true iron deficiency. 2, 3

• Faecal occult blood testing is of no benefit in the investigation of iron deficiency anemia and should not be ordered. 1

When to Pursue Additional Workup

Gastrointestinal Investigation

• Upper and lower GI investigations should be considered in all postmenopausal women and all men with confirmed iron deficiency anemia unless there is significant overt non-GI blood loss. 1

• Colonoscopy or CT colonography is preferred over barium enema for lower GI evaluation. 1

Hematology Referral Indicators

• Urgent hematology referral is indicated if the peripheral smear shows blasts or dysplastic cells, if pancytopenia persists without reversible cause, if reticulocyte count remains inappropriately low after nutritional repletion, or if there is no improvement after 3 months of targeted therapy. 2

• Consider hematology consultation if iron studies are inconsistent with iron deficiency, if concurrent cytopenias appear, or if hemoglobinopathy is suspected. 3

Nephrology Evaluation

• Nephrology referral is appropriate when elevated creatinine or reduced eGFR indicates chronic kidney disease as the underlying driver of normocytic anemia. 2