DOH exam preparation (Abu Dhabi) – page 13

Adenine allows red cells to resynthesize ATP, improving post-transfusion survival and extending storage. Citrate, not adenine, prevents clotting by binding calcium.

Red cells are stored refrigerated at 1–6 °C. The wider 1–10 °C range applies to transport, not storage.

Storage at 20–24 °C allows skin bacteria to multiply, unlike refrigerated red cells or frozen plasma.

Emergency release requires the requesting physician to document that the clinical situation justified transfusion before testing was finished. Testing is completed afterwards and abnormalities reported at once.

A standard 450–500 mL collection is limited to donors weighing at least 50 kg (110 lb). Lighter donors need a reduced-volume collection with less anticoagulant.

Toxoids and inactivated vaccines do not cause deferral in symptom-free donors. Live attenuated vaccines such as varicella, oral typhoid and yellow fever require a temporary deferral of 2–4 weeks.

Lewis antigens are made in tissues and taken up passively onto red cells from plasma. This is why the Lewis type can change, e.g. in pregnancy. Non-secretors with a Le gene still express Lea.

Antenatal RhIG does not replace the postpartum dose. After a D-positive birth she needs a fetomaternal hemorrhage screen and at least one dose.

Each red cell unit contains about 200–250 mg of iron, which the body cannot excrete. Ferritin is used to monitor iron load and guide chelation therapy.

Anti-A and anti-B are mainly IgM and activate complement through C9, lysing cells in the circulation. This can lead to shock, DIC and acute kidney injury.

Only the packed red cell column should be read; the buffy coat of white cells and platelets adds to the reading. A falsely high Hct would lower the calculated MCHC, not raise it.

The G6PD gene lies on the X chromosome, so males are usually affected and females are carriers, although some heterozygous females are affected due to lyonization. Pyruvate kinase deficiency, not G6PD deficiency, is autosomal recessive.

Typical hemolytic uremic syndrome follows infection with Shiga toxin-producing E. coli such as O157:H7, with toxin damaging renal endothelium. C. difficile causes colitis but not classic HUS.

With no functional alpha genes, the fetus makes only gamma-4 tetramers (Hb Bart's), which cannot deliver oxygen, leading to hydrops. The --/-α genotype causes HbH disease, which is compatible with life.

The aPTT uses phospholipid (partial thromboplastin) plus an activator such as silica, kaolin or ellagic acid, then calcium. Tissue factor is the PT reagent.

MCHC (g/dL) = Hb (g/dL) × 100 ÷ Hct (%) = 1200 ÷ 36 = 33.3 g/dL (333 g/L), which is within the normal range.

Normal discocytes show central pallor of about one third of the diameter. Pallor larger than this indicates hypochromia, and a thin rim of hemoglobin is seen in severe hypochromia.

Adult T-cell leukemia/lymphoma is caused by HTLV-1 and shows CD4+ 'flower cells' and hypercalcemia. EBV is linked to Burkitt and Hodgkin lymphoma.

Monocytes have folded nuclei, lacy chromatin and grey-blue cytoplasm with fine azurophilic dust and vacuoles. Reactive lymphocytes have deeper blue cytoplasm and denser chromatin.

Postzone is antigen excess; prozone is antibody excess. Maximum lattice formation and precipitation occur at the zone of equivalence.