🧪 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: men 14 to 18, women 12 to 16; Hct about 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 combined percentage of neutrophils and bands, divided by 100; under 1,500/mm³ is neutropenia, under 500/mm³ is severe, and neutropenic precautions usually start below about 1,000/mm³ 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): 42 to 52% in men, 37 to 47% in women, about three times hemoglobin; rises with dehydration, falls with fluid overload despite unchanged red cell mass, and falls with bleeding, though acute bleeds may look normal until fluid shifts or IV fluids dilute it.
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, whereas in anemia of chronic disease the iron also falls but the TIBC is normal or low.
Iron supplements: Take oral iron on an empty stomach with vitamin C, away from antacids; it darkens stools and constipates; sip liquid iron through a straw to avoid staining teeth; lock all iron away since child overdose is deadly (antidote deferoxamine); give IM iron 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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