haemolytic disease of newborn

Question 1

Describe the pathophysiology of HDN.

Answer 1

• HDN / HDNB / HDFNB (F= foetus)
• jaundice: elevated (>300 uM) &/or rapid inc. (>8.5 uM/hr) serum bilirubin w/in 24hr or 14+ days post partum
• Positive DAT: maternal Ab bound on Bb’s RBC

Question 2

Which antibodies commonly cause HDN?

Answer 2

• IgG Ab (reactive in IAT)
• Rhesus: anti -D, -c, -C, -e, -E
• Kell: anti-K*, -k
• Duffy; Anti- Fya
• MNS, Kidd
• ABO: common w/ Anti- A, B

Question 3

Describe how an antibody titre is determined. & clinically significant results

Answer 3

• diluted plasma reacted against cells w/ homozygous expression of Ag present
• titre = reciprocal of highest dilution w/ 1+ reactivity
• CSig.: ≥32 titre or rises by 2 dilutions

Question 4

Summarise the serological testing required during the ante and post natal periods.

Answer 4

• Ante-: @ first trimester visit: ABO & Rh(D) typing & Ab screen
• Retest in 28 weeks for ABO & Rh(D) type and Ab screen if neg.
• Post-:
  • Mum have ABO & Rh(D) and Ab screen (if not done already)
  • Bb: if mum is Rh(D) neg or has CSig. Ab = do ABO/Rh(D) group, DAT, Hb, Bilirubin on baby
• IF DAT positive => Ab screen => Ab ID = History / phenotype / Aby quant. (every 4 wks till 28 wks, every 2 wks)

Question 5

Explain how Rh(D)Ig is used in the prevention of HDN. Include the challenges faced in Rh(D)Ig administration.

Answer 5

• giving Rh(D)Ig Ab to Rh(D) neg mum => dec. incidence of RHD
• bc Rh(D) pos Ag on Bb’s cells are removed before being detected by the mum’s ISys. ≠ make immune anti-D Ab
• Challenges: Collect sample for Ab screen THEN give Rh(D)Ig bc Can’t distinguish from immune anti-D => complicate test (FPos)

Question 6

Mechanism of HDN

Answer 6

1. Ag neg mum carrying a Ag pos child (inherited from dad)
2. Mum is exposed to Bb’s Ag on RBC (pos)
3. Mum’s ISys stimulated => IgG Ab produced
4. Mum’s IgG crosses placenta & enters Bb’s circulation => Ab bind to Ag on Bb’s RBC
5. Ab coated RBC destroyed by foetal reticuloendothelial sys.

Question 7

Describe the tests for foetal anaemia (before & current)

Answer 7

• Before: amniocentesis & cordocentesis = invasive -> can lose foetus
• Current: Middle cerebral artery (MCA) Doppler ultrasound= measures velocity of blood flow. If inc. = anaemia

Question 8

Describe the tests for foetomaternal haemorrhage

Answer 8

• Acid elution / Kleihauer / - Betke test: Expose blood film to acid = denatures HbA (adult) => stain intact HbF (foetus) w/ erythrosine / eosine
• Flow cytometry: detect D pos cells in maternal blood using Fluorescent-labelled anti-D Ab

Question 9

discuss treatment options for HDNB

Answer 9

• Phototherapy: use blue light (460nm) => break down bilirubin (can be effective as Xchange transfusions)
• Exchange transfusions: exchange foetal blood => remove Ab & replace RBC

Question 10

Understand the rationale and use of prophylactic anti-D

Answer 10

• Prophylactic anti-D given to mums that don’t produce anti-D Ab
• to prevent mum being exposed to Bb’s Rh(D) pos cells = prevent sensitisation of mum i.e prevent mum’s ISys. from being stimulated = no Ab & no memory B cells made

Question 11

Kleihauer test equation to calculate foetal RBC (ml) =

And requirements

Answer 11

Foetal RBC = ({total} Foetal cells per FOV ÷ {total} Adult cells per FOV) x 2400

• 100 adult cells in 40x FOV
• Count # of foetal RBC w/ ≥10k maternal cells (5-10+ 40x FOV)

Question 12

Flow cytometry equation to determine foetal RBC (mL) =

And Control

Answer 12

Foetal cells RBC = % foetal cells x 21.96

• Neg cntrl: Adult blood
• Pos cntrl: 0.25% cord Rh(D) pos cells in Adult Rh(D) neg cells