Fetal blood sampling and scalp lactate — structured oral station (12 minutes)

Opening prompt — "The kit is ready. Are you going to sample?"

Model response — earn the right to the procedure first: [1][5]

• "Before I open the pack I ask one question: will the result change what I do? If I would deliver whatever the number said, the sample is a delay, not a decision."
• "So I confirm the reversible causes are corrected — oxytocin off, full lateral, fluid given, blood pressure corrected, cord prolapse excluded — and I check that the trace is stable rather than deteriorating."
• "Then I run the contraindication list, confirm membranes are ruptured, the cervix is more than 4 cm and the presentation is cephalic, and I consent her."[1][5]

Examiner is listening for: the will-it-change-management question, resuscitation before investigation, and the safety checks unprompted. [1]

Probe 1 — "List the contraindications."

• Gestation under 34 weeks.
• Serious sustained compromise on the CTG, for example bradycardia beyond 5 minutes.
• Known or suspected fetal bleeding disorder — haemophilia, alloimmune thrombocytopenia.
• Non-vertex presentation.
• Maternal HIV, hepatitis B or hepatitis C.
• Active genital herpes lesions.
• Suspected intrauterine sepsis.[1][7]

Add the discriminator unprompted: "Group B streptococcus colonisation is not a contraindication."[1]

Probe 2 — "Describe the procedure."

• "Left lateral position, or lithotomy with a lateral wedge — I avoid flat supine because aortocaval compression can worsen the very trace I am investigating."[1]
• "Amnioscope with a light source, swabs, gel, ethyl chloride spray, guarded blade, heparinised capillary tube, and the analyser checked and ready before I start."
• "I seat the amnioscope on the scalp and confirm I am on scalp, not fontanelle, face or genitalia. I clean and dry, apply gel so the blood beads rather than runs, stab with the guarded blade, and collect by capillary action without air bubbles."
• "Then pressure through a contraction until haemostasis, confirm the fetal heart rate, analyse immediately, and document."[1][2]

Probe 3 — "Give me the numbers."

• "pH: 7.25 or more normal; 7.21 to 7.24 borderline, repeat in 20 to 30 minutes; 7.20 or less abnormal, expedite birth."[1]
• "Lactate: 4.0 mmol/L or less normal; 4.1 to 4.7 mmol/L borderline, repeat in 20 to 30 minutes; 4.8 mmol/L or more abnormal, expedite birth."[1]
• "If the result is normal but the CTG stays abnormal, I repeat in 30 minutes. If two successive results are stable I defer further testing unless new abnormal features appear."[1]
• The caveat that marks you out: "Lactate cut-offs are device-specific. The classic threshold above 4.8 mmol/L belongs to a meter no longer manufactured, and prospective data using the StatStrip system support around 5.2 mmol/L. I would quote our unit's meter and protocol."[4]

Probe 4 — "Your pH is 7.27. The baseline has risen another 10 beats and variability is now 3 bpm. What do you do?"

• "The number is normal and the physiology is not. A sample is a single time point that lags the trace, and this fetus has a rising baseline with falling variability, which is failing central compensation."[1][2]
• "I would not treat 7.27 as permission to wait. I would escalate: consultant to the bedside, continue resuscitation, alert theatre, and expedite birth if there is no rapid improvement."[1]
• "The commonest way this test causes harm is not the needle. It is a reassuring digit used to justify delay."[5][1]

Probe 5 — communication. "She asks what it involves and whether it will hurt her baby."

Consent in real words, at her level, in labour. [1]

• "We would like to take a tiny drop of blood from your baby's head to check how the baby is coping with labour. It means an internal examination with a small tube, and a quick scratch on the baby's scalp — about the size of a pinprick."
• "It usually takes ten to fifteen minutes with the set-up, and it can be uncomfortable for you, so tell me if you need me to stop."
• "Most babies have a small mark that heals on its own. Rarely there can be a small cut or an infection at the spot, and the paediatric team will check it after birth."
• "If the result is reassuring we keep going. If it is not, we would recommend delivering your baby quickly, most likely by caesarean at this stage of labour. If at any point I think your baby needs to be born now, we will not wait for the test."[1][2]

Probe 6 — "Should we still be doing this test at all? Argue both sides."

• For: "It is the only bedside test that quantifies fetal acid-base status. It can avert an unnecessary caesarean when the trace looks worse than the fetus, and serial values give me a trend rather than a snapshot."[5][2]
• Against: "It is invasive, it costs fifteen to twenty minutes, roughly a fifth of pH attempts fail, its cut-offs migrate with the hardware, and there has never been randomised evidence that it improves neonatal outcome. Only one trial ever compared CTG plus sampling with CTG alone, in the 1970s, and it was underpowered. Recent trials designed to answer the question could not recruit."[5][2]
• My practice: "I use it selectively — a stable but persistently abnormal trace where I genuinely might continue labour — and I use scalp stimulation first, because it is instant and non-invasive, while accepting that agreement between the two is only fair."[6][5]

Probe 7 — "Lactate or pH? Justify."

• "Either is acceptable under the ANZ guideline. In the randomised multicentre trial, metabolic acidaemia at birth occurred in 3.2 per cent with lactate and 3.6 per cent with pH — no significant difference."[3]
• "The real advantage is sampling success: 98.7 per cent for lactate versus 79.4 per cent for pH in the Cochrane data, because lactate needs only a few microlitres."[2]
• "So I would choose lactate for feasibility, not for accuracy — and I would make sure the meter is the one our thresholds were written for."[4][3]