One lavender tube gives you a census of every cell in the blood. This page teaches the
red cell line — hemoglobin, hematocrit and the indices that tell you why the anemia happened —
and platelets. The white cell differential and the coagulation panel get their own page
(NG-153), because they answer completely different questions.
🔬 MCV tells you WHYSmall = iron. Big = B₁₂ or
folate. Normal size but too few = bleeding or a failing kidney.
🩹 PLT 150–400Under 50 bleeds with
minor trauma. Under 20 bleeds with none. Platelets PLUG.
⚠️ Read this before you memorize a single number. Every value on this page is a typical adult reference range — ranges vary by laboratory, so always use the range printed on your patient’s result. Critical-value cut-offs vary even more: every facility publishes its own critical list and its own call-back policy. Learn the direction and the meaning; verify the exact digits locally.
🧪
PART 1 · WHAT COMES BACK
THE REPORT
One tube, one printout — and a habit of reading it as a whole.
Figure 1 — a real report reads as a story. The flag column is the analyzer’s arithmetic; the interpretation is yours.
🧪 The four headline numbers
WBC4.5–11.0 ×10⁹/Lthe defense — infection (see NG-153)
Hemoglobin12–18 g/dLthe oxygen carrier
Hematocrit36–54 %how much of the blood is red cells
Platelets150–400 ×10⁹/Lthe plug — bleeding
Typical adult reference ranges. Hemoglobin and hematocrit differ by sex (roughly 12–16 g/dL and 36–48% for women, 14–18 g/dL and 42–54% for men) — always use the range printed on your patient’s result.
🧠 W-H-H-P: White, Hemoglobin, Hematocrit, Platelets. Four numbers, four jobs: fight, carry, thicken, plug.
🧪 The other values on the same tube
Value
Typical range
What it tells you
RBC count
4–6 million/µL
How many red cells there are
MCV
80–100 fL
Average red cell SIZE — the value that names the anemia
MCH / MCHC
per lab
How much hemoglobin is in each cell (how pale the cells look)
RDW
per lab
How uneven the cell sizes are — rises early in iron deficiency
Differential
%
Which white cells — the whole of NG-153
🧠 MCV names the anemia. RDW says it is new. A rising RDW often shows up before the MCV has finished moving.
🩸 Collecting the CBC properly
Lavender (EDTA) tube, filled to the line, then inverted gently 8–10 times. Never shake it.
A clotted lavender tube gives a falsely LOW platelet count — the platelets are stuck in the clot instead of being counted.
A hemolysed sample invalidates the count and falsely raises potassium on any chemistry drawn with it.
Never draw above a running IV — the dilution drags the H&H down.
🧠 Invert, do not shake. Shaking bursts the cells you were sent to count.
🦴
PART 2 · WHERE THE CELLS COME FROM
THE FACTORY
One stem cell, three lines — and the four ingredients the red line needs.
Figure 2 — one stem cell in the marrow becomes all three lines. That is why one marrow insult can drop all three counts at once.
🦴 Where blood cells are made
All three blood cell lines come from one stem cell in the red bone marrow.
A single hematopoietic stem cell can become a red cell, any white cell, or a platelet. The marrow makes them, the vessels carry them away, and the spleen and liver eventually break them down.
Red cells live about 120 days
Platelets live about 8–10 days
Neutrophils live hours to days
🧠 One factory, three products. If all three counts are low, look at the factory, not at three separate problems.
🚨 PANCYTOPENIA — all three lines down at once
What it looks like: low red (tired, pale), low white (infections), low platelets (bruising and bleeding) — together.
Causes to expect: chemotherapy, radiation, leukemia, aplastic anemia, some drugs, severe B₁₂ or folate deficiency.
What you do: combine neutropenic precautions and bleeding precautions, plan rest between activities, and monitor for fever with the same urgency as for bleeding.
🧠 Three lines down = one factory problem. Ask what has hit the marrow.
💊 What the red line actually needs to be built
Erythropoietin from the kidney — the signal. A failing kidney stops sending it, which is why chronic kidney disease causes anemia.
Iron — to build the hem that holds the oxygen.
Vitamin B₁₂ and folate — to build the DNA so the cell can divide.
Intrinsic factor from the stomach — without it B₁₂ cannot be absorbed (pernicious anemia; also after gastric surgery).
🧠 Signal, brick, blueprint. EPO is the signal, iron is the brick, B₁₂/folate is the blueprint. Miss any one and the line fails in its own particular way.
🩸
PART 3 · THE RED LINE
CARRY THE OXYGEN
Hemoglobin, hematocrit and the red cell count — three views of the same job.
Figure 4 — the cell, its cargo and the iron at the center of it. Every red-cell problem on this page is a failure of one part of this drawing.
🩸 HEMOGLOBIN — the number that decides transfusion
Typical adult roughly 12–18 g/dL overall — about 12–16 for women and 14–18 for men.
12–18NORMAL
Oxygen delivery is adequate.
8–11RISKY
Report it — especially before surgery or a procedure. Think bleeding, anemia, malnutrition, cancer.
under 7TRANSFUSE
Expect a transfusion order in most adult situations. Symptomatic clients may be transfused higher.
Top signs of a low hemoglobin: pale skin and dusky tones · cool, clammy skin · fatigue and weakness · tachycardia · breathless on effort.
🧠 “Below 7 → heaven or a transfusion.” Blunt, but you will never forget the number.
Figure 3 — spin the tube and the hematocrit is simply the height of the red column as a percentage. This is why dehydration raises it without adding a single cell.
📏 HEMATOCRIT — and the rule of three
Typical adult roughly 36–54%, varying by sex. It is the percentage of the blood volume that is red cells.
Hct ≈ 3 × Hgb. Learn the three pairs: 12/36 · 15/45 · 18/54.
HIGH HctDehydration first. Also chronic hypoxia, smoking, high altitude, polycythemia. “High labs = dry body.”
LOW HctBleeding · anemia · malnutrition — and fluid overload, which dilutes it without losing any cells.
🧠 The hematocrit is a ratio, so it moves when the WATER moves — not only when the cells do. That single idea explains both ends of the scale.
Figure 5 — hemoglobin and hematocrit as two axes, with the action written under the zone that triggers it.
⚠️ The trap: an acute bleed can have a NORMAL H&H
In the first hours of a hemorrhage the H&H can look normal. Cells and plasma leave together — so the ratio has not changed yet and the hemoglobin can look reassuring.
The H&H falls later, as fluid shifts in (or is infused) and dilutes what is left.
Never let a normal H&H reassure you in a client who is actively bleeding. Watch the vital signs, level of consciousness, urine output and the dressing.
🧠 The H&H reports the past, the vital signs report the present. In a fresh bleed, trust the patient.
🔴 RBC COUNT — the third red number
Typical adult 4–6 million/µL.
Low — anemia, renal failure (no erythropoietin), bleeding, marrow failure.
High — dehydration first; then chronic hypoxia (COPD, heart disease, high altitude, smoking) and polycythemia vera.
Treatments that follow the cause: iron, B₁₂/folate, erythropoietin-stimulating agents in kidney disease, and transfusion when the hemoglobin demands it.
🧠 “High labs = dry body.” When RBC, Hgb, Hct and BUN all rise together, think dehydration before you think disease.
😮💨
PART 4 · ANEMIA BY SIZE
SMALL · NORMAL · BIG
The MCV turns “anemia” into a diagnosis with a treatment attached.
Figure 6 — the MCV sorts anemia into three boxes, and each box has its own cause and its own treatment. This is the highest-yield figure on the page.
🔬 MICROCYTIC — small and pale
MCV under 80 fL. The cell could not fill up.
Commonest cause: iron deficiency — chronic blood loss (GI bleeding, heavy menstrual periods), poor intake, malabsorption. Also thalassemia.
Teaching: iron-rich foods (red meat, liver, beans, leafy greens, fortified cereals); take iron with vitamin C to improve absorption; on an empty stomach if it is tolerated; expect black stools and constipation; use a straw for liquid iron to protect the teeth.
🧠 Small cells = missing IRON. Then ask the real question: where is the client losing blood?
🔬 NORMOCYTIC — normal size, too few
MCV 80–100 fL. The cells that exist are fine — there are not enough of them.
Causes: acute blood loss (the cells left whole), anemia of chronic disease, and chronic kidney disease — a failing kidney stops making erythropoietin, so the marrow is never told to build.
Treatment follows the cause: stop the bleeding, treat the disease, or give an erythropoietin-stimulating agent per order in kidney disease.
🧠 Normal size, low count → look at the kidney or look for blood.
🔬 MACROCYTIC — big and immature
MCV over 100 fL. The cell grew but could not divide, because DNA synthesis failed.
The discriminator: only B₁₂ deficiency adds neurological signs — numbness and tingling in the hands and feet, unsteady gait, memory change. Folate deficiency does not.
🧠 Big cells + numb feet = B₁₂. Big cells with normal sensation = folate. Pernicious anemia needs lifelong B₁₂ injections, not tablets.
Figure 7 — the same laboratory value, two opposite patients. The magnified capillary window is the reason each set of signs happens.
😮💨 Nursing an anemic client
Pace the care. Rest between activities; cluster interventions; let them recover before the next one.
Fall precautions — dizziness on standing is common.
Monitor heart rate, respiratory rate and oxygen saturation with activity, not just at rest.
Oral care with a soft brush; mucous membranes are fragile.
Watch for the cause: stools for blood, menstrual history, diet, drugs (NSAIDs, anticoagulants).
🧠 Assess the client on the move. A resting anemic client can look fine; walk them to the door and the truth appears.
🚨 POLYCYTHEMIA — the opposite problem
Too many red cells makes the blood thick and slow — the risk is a CLOT, not a bleed.
Ruddy, flushed face · headache · dizziness · visual changes
Itching, classically after a hot shower
Splenomegaly and a feeling of fullness
Complications: DVT, stroke, myocardial infarction
Care: encourage fluids, keep them mobile, avoid prolonged sitting, watch for chest pain, calf pain and neuro changes. Expect therapeutic phlebotomy per order.
🧠 Thick blood clots. Everything you do for polycythemia is aimed at keeping it moving and keeping it diluted.
🩹
PART 5 · PLATELETS
PLUG THE BLEEDING
The last of the three lines — and the step before the clotting factors take over.
Figure 8 — three steps in order. Step 2 is what the platelet count measures; step 3 is what PT/INR and aPTT measure. Two different numbers, two different steps.
Destruction — ITP, DIC, sepsis, an enlarged spleen sequestering them.
Drugs — heparin (HIT), some antibiotics, some anticonvulsants.
Dilution — massive transfusion of stored blood and fluids.
Heparin-induced thrombocytopenia (HIT) is the one to know: the platelet count falls sharply about 5–10 days into heparin therapy, and the danger is clotting, not bleeding. Stop the heparin; a non-heparin anticoagulant is used instead.
🧠 Remember: aspirin and clopidogrel do not lower the platelet count — they stop the platelets you have from sticking. The number looks normal; the function is not.
✅ Bleeding precautions — the practical list
Soft toothbrush; no flossing if the count is very low
Electric razor only
Avoid IM injections and, where possible, invasive procedures
Hold pressure longer after every needle stick
Fall precautions; non-slip footwear; keep the path clear
No NSAIDs or aspirin unless specifically ordered
No rectal temperatures, suppositories or enemas
Stool softeners to prevent straining
🧠 Soft brush, electric razor, no rectal anything. Three habits cover most of the exam questions and most of the real risk.
🧠 Where each number lives in the clotting story
Step
What happens
Which lab measures it
1 · Vasoconstriction
The torn vessel clamps down
None — this is the muscle wall
2 · Platelet plug
Platelets stick, activate and pile up
Platelet count (and bleeding time / function tests)
3 · Fibrin mesh
The clotting factors cement the plug
PT/INR and aPTT — see NG-153
🧠 A client can bleed with a perfect platelet count if the factors are the problem — and vice versa. Always ask which step is broken.
🎯 Cover & check — 8 rapid-fire questions
Q1. Hemoglobin 6.4 g/dL in a stable adult. What do you expect the provider to order?Show answerA transfusion. Under 7 g/dL is transfusion territory in most adult situations.
Q2. A client with GI bleeding has a hematocrit of 30%. What is your action?Show answerReport the abnormally low level. 30% is well below the 36–54% range in a client who is actively bleeding.
Q3. Hemoglobin 14. What hematocrit would you expect?Show answerAbout 42% — the rule of three, Hct ≈ 3 × Hgb.
Q4. MCV 71 with a high RDW. Which anemia, and which teaching?Show answerMicrocytic — iron deficiency. Iron with vitamin C, expect black stools and constipation, and find the source of the blood loss.
Q5. MCV 112 with numbness and tingling in the feet. Which deficiency?Show answerVitamin B₁₂. Folate deficiency gives the big cells but not the neurological signs.
Q6. A client with chronic kidney disease is anemic. Why, and what size are the cells?Show answerThe failing kidney makes too little erythropoietin, so the marrow is not told to build. The cells are normocytic — normal size, just too few.
Q7. Platelets 42,000/mm³. Which precautions go up?Show answerBleeding precautions — soft toothbrush, electric razor, no IM injections, no rectal temperatures or suppositories, fall precautions, hold pressure after sticks.
Q8. A client on a heparin drip has a platelet count that has halved since day 6. What are you thinking?Show answerHeparin-induced thrombocytopenia (HIT). Stop the heparin and notify the provider — the danger is clotting, not bleeding.