🧪 Lab Values · Lesson 1 of 5 · 6 min
The CBC: Hemoglobin, White Cells and Platelets
🩸 A hemoglobin of 7 means a transfusion conversation. A white count of 22,000 with 20% bands means a serious infection. A platelet count of 18,000 means a brain bleed is one fall away. Three numbers, three plans.
💡 Key idea
Hgb 12 to 18, Hct three times that. WBC 5 to 10 thousand; bands mean acute infection. Platelets 150 to 400 thousand; below 50 thousand, bleeding precautions; below 20 thousand, danger.
🧠 Why it matters
HEMOGLOBIN: men about 14 to 18 g/dL, women 12 to 16; HEMATOCRIT: men 42 to 52%, women 37 to 47%, roughly three times the hemoglobin. Low values mean anemia or bleeding; transfusion is often considered below 7 to 8 g/dL depending on symptoms and cardiac history. Hematocrit rises with dehydration and falls with fluid overload even when red cell mass is unchanged. WHITE BLOOD CELLS: 5,000 to 10,000/mm³; high counts suggest infection, inflammation, steroids or leukemia; low counts (leukopenia) follow chemotherapy, viral illness or marrow failure. A SHIFT TO THE LEFT means an increase in immature neutrophils (BANDS), a sign of acute bacterial infection. The ABSOLUTE NEUTROPHIL COUNT is the WBC multiplied by the percentage of neutrophils plus bands; below 1,500 is neutropenia and below 500 is severe; neutropenic precautions usually start below about 1,000, per facility policy. PLATELETS: 150,000 to 400,000/mm³; bleeding precautions begin below about 50,000, and spontaneous bleeding risk rises below 20,000. Red cell indices: a low MCV suggests iron deficiency; a high MCV suggests vitamin B12 or folate deficiency. ERYTHROPOIETIN is given for the anemia of chronic kidney disease; iron is taken with vitamin C, away from dairy, and it blackens stools.
🏥 In the real world
🔬 A post-op client's hemoglobin falls from 13.2 to 8.9 overnight with a heart rate of 112. Nothing is on the dressing. The nurse reports the trend, not just the number: a 4-gram drop in 12 hours is bleeding somewhere until proven otherwise. The CT shows a retroperitoneal hematoma. The lab trend found what the dressing could not.
📌 Takeaways
- Hemoglobin and hematocrit both rise with dehydration and fall with fluid overload or bleeding
- Left shift means bands, means bacterial infection
- ANC under 1,500 is neutropenia, under 500 severe; platelets under 50,000 need bleeding precautions
📖 Terms in this lesson
Shift to the left: A rise in immature neutrophils (bands) in the blood, a sign of acute bacterial infection.
Absolute neutrophil count (ANC): White cells times the percentage of neutrophils and bands; under 1,500 is neutropenia, under 500 is severe, and precautions usually start below about 1,000 per policy.
Red blood cell count (RBC): About 4.7 to 6.1 million/mm³ in men and 4.2 to 5.4 million/mm³ in women; low means anemia or bleeding, high means dehydration, chronic hypoxia or polycythemia.
Hemoglobin (Hgb): About 14 to 18 g/dL in men and 12 to 16 g/dL in women; a transfusion is usually considered below 7 to 8 g/dL, and below about 6 to 7 is a critical value.
Hematocrit (Hct): About 42 to 52% in men and 37 to 47% in women, roughly three times the hemoglobin; it rises with dehydration and falls with bleeding or fluid overload even when red cell mass is unchanged.
White blood cell count (WBC): Normal 5,000 to 10,000/mm³ (some labs 4,500 to 11,000); high means infection, inflammation, steroids or leukemia, low follows chemotherapy or marrow failure, and below about 2,000 or above about 30,000 is critical.
Platelets (PLT): Normal 150,000 to 400,000/mm³; bleeding precautions start below about 50,000, spontaneous bleeding risk climbs below 20,000, and below about 10,000 to 20,000 is a critical value.
Mean corpuscular volume (MCV): The average red cell size, normally about 80 to 100 fL; a low MCV points to iron deficiency, a high MCV to vitamin B12 or folate deficiency.
Ferritin: The body's iron store, about 12 to 300 ng/mL in men and 12 to 150 ng/mL in women; low confirms iron-deficiency anemia, while it rises with inflammation, so a normal value does not rule deficiency out.
Serum iron and TIBC: Serum iron about 60 to 170 mcg/dL and total iron-binding capacity about 250 to 450 mcg/dL; in iron deficiency the iron falls while the TIBC rises, the opposite of anemia of chronic disease.
Iron supplements: Oral iron is taken on an empty stomach with vitamin C and away from antacids, turns stools dark, constipates, stains teeth unless sipped through a straw, and is locked away because a child's overdose is deadly (antidote deferoxamine); IM iron goes in by Z-track.
✅ Test yourself
A client's WBC count is 8,000 with 50% neutrophils and 10% bands. What is the absolute neutrophil count?
- 4,800
- 4,000
- 8,000
- 400
Answer: A · 4,800
ANC = WBC × (neutrophils + bands) = 8,000 × 0.60 = 4,800.
Which laboratory value requires the nurse to institute bleeding precautions?
- Platelets 35,000/mm³
- Hemoglobin 15 g/dL in a male client
- WBC 7,500/mm³
- Hematocrit 44%
Answer: A · Platelets 35,000/mm³
Platelet counts below about 50,000 increase bleeding risk; below 20,000, spontaneous bleeding can occur.
A client with pneumonia has a WBC of 19,000 with an increased number of bands. What does this 'shift to the left' indicate?
- An acute bacterial infection
- Bone marrow suppression
- Allergic reaction
- A viral infection such as influenza
Answer: A · An acute bacterial infection
Bands are immature neutrophils released early in response to acute bacterial infection; viral illness and marrow suppression lower the count rather than shifting it left.
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