PA13.1-3 | Hematopoiesis & Blood Specimen Basics — Summary & Reflection

REFLECT

Take 3 minutes and think through this clinical scenario:

A 45-year-old woman is admitted with easy bruising and gum bleeding for 6 weeks. Her GP sent a CBC last week (Hb 10.2, WBC 3.1, Plt 28,000). You are the Pathology registrar reviewing the peripheral smear. The smear was prepared from a green-top (heparin) tube because the purple-top was out of stock at the clinic.

Consider:
1. How does the anticoagulant in the tube affect what you are able to reliably conclude from this smear?
2. What specific morphological information will be distorted or unreliable?
3. If you were to call the clinic, what would you tell them — and in what order of priority?
4. Separately: this patient's platelet count is 28,000. If she also had a prolonged aPTT in the same blood draw — would you trust that result? Why or why not?

There is no single right answer. The goal is to connect what you just learned (tube science) to real clinical decision-making under imperfect conditions.

KEY TAKEAWAYS

What you have learned in this module:

Haematopoiesis (PA13.1):
• All blood cells descend from the haematopoietic stem cell (HSC) — self-renewing, multipotent
• HSC → CMP (red cells, platelets, granulocytes, monocytes) and CLP (T, B, NK cells)
• Key drivers: EPO (erythropoiesis, kidney-derived), TPO (megakaryopoiesis), G-CSF (granulopoiesis)
• Erythroid sequence: proerythroblast → basophilic → polychromatic → orthochromatic erythroblast → reticulocyte → mature RBC (7 days, nucleus extruded at orthochromatic stage)
• Reticulocyte count = real-time marrow activity meter (high = responding, low = failing)
• In foetal life: yolk sac → liver + spleen → bone marrow. Liver/spleen retain haematopoietic potential.
• Extramedullary haematopoiesis (EMH): reactivation of liver/spleen haematopoiesis in marrow failure/overload. Classic causes: beta-thalassaemia major, myelofibrosis. Smear shows leukoerythroblastic picture (NRBCs + immature granulocytes + teardrops). Skull X-ray: 'hair-on-end' in thalassaemia.

Blood specimen collection (PA13.3):
• Tube colour = additive = specific test; using the wrong tube corrupts the result
• Purple (EDTA): CBC, PBF, retics, HbA1c — calcium chelation, irreversible
• Blue (citrate 3.2%): coagulation screen (PT, aPTT, INR) — must fill to 90%+
• Green (lithium heparin): electrolytes, LFTs, RFTs — activates antithrombin
• Red/Gold: serology, biochemistry — allows clot formation
• Grey (fluoride-oxalate): glucose, lactate — blocks glycolysis
• Order of draw: BC → Blue → Red → Green → Purple → Grey
• Smear quality: wedge technique, 30–45° angle, air-dry immediately, read in the monolayer zone