Normal Values & Critical Labs

⚠ Critical Values — Automatic Escalation Required

K⁺

<2.5 or >6.5 mmol/L

Na⁺

<120 or >155 mmol/L

Glucose

<3 or >25 mmol/L

Lactate

>4 mmol/L

<7.2 or >7.55

<70 g/L

Neutrophils

<0.5 ×10⁹/L

Platelets

<20 ×10⁹/L

PaO₂

<8 kPa

PaCO₂

>6.5 + acidosis

OSCE Escalation Script

"This result is a critical abnormality. I would escalate immediately to my senior doctor or the ICU team and initiate appropriate management now."

Top 6 PLAB 2 Lab Abnormalities

#	Abnormality	Key Tests	Immediate Action
1	Hyperkalaemia	K⁺, ECG	IV calcium gluconate → insulin-dextrose → sodium bicarbonate if acidotic
2	Sepsis / elevated lactate	Lactate, VBG, blood cultures	Sepsis Six within 1 hour — escalate to senior/ICU
3	DKA (glucose + ketones)	Glucose, VBG, ketones, K⁺	Fluids → check K⁺ → fixed-rate insulin. Check K⁺ before insulin.
4	Anaemia (low Hb)	FBC, MCV, reticulocytes	Hb <70 = transfusion threshold. Check MCV for type.
5	AKI (raised creatinine)	Creatinine, U&E, eGFR, urine output	Stop nephrotoxins, fluid status, senior review if severe
6	Hyponatraemia	Sodium, osmolality, fluid status	Slow correction — rapid correction causes osmotic demyelination

Haematology Reference Ranges

Test	Normal Range	Clinical Note
Haemoglobin (M)	130–170 g/L	Below = anaemia. Check MCV for type.
Haemoglobin (F)	115–155 g/L	Pregnancy norm: 100–120 g/L (haemodilution)
WCC	4–11 ×10⁹/L	↑ infection/inflammation · ↓ bone marrow or viral
Neutrophils	2–7.5 ×10⁹/L	<0.5 = neutropenic sepsis protocol
Platelets	150–400 ×10⁹/L	<20 = critical · <50 = bleeding risk during procedures
MCV	80–100 fL	Low = iron/thal · High = B12/folate/alcohol/liver
INR	0.9–1.2	Target 2–3 (warfarin AF) · 2.5–3.5 (mechanical valve)
APTT	25–35 seconds	Prolonged = haemophilia, heparin, lupus anticoagulant

PLAB 2 tip: Low Hb + low MCV = iron deficiency anaemia. Low Hb + high MCV = B12/folate deficiency or alcohol. Corrected calcium = Ca + 0.02 × (40 − albumin).

Biochemistry Reference Ranges

Test	Normal Range	Clinical Note
Sodium	135–145 mmol/L	Fluid balance marker · <120 or >155 = critical
Potassium	3.5–5.0 mmol/L	Cardiac rhythm · <2.5 or >6.5 = critical
Urea	2.5–7.0 mmol/L	Hydration + renal function. High = GI bleed or dehydration
Creatinine	60–110 µmol/L	Kidney function — rises late in AKI. Check trend.
eGFR	>90 mL/min	CKD staging: <60 = CKD3 · <30 = CKD4 · <15 = CKD5
Glucose (fasting)	3.9–5.5 mmol/L	Diabetes: fasting ≥7.0 · OGTT 2h ≥11.1
HbA1c	<48 mmol/mol	Pre-diabetes 42–47 · Diabetes ≥48
CRP	<5 mg/L	Infection/inflammation marker
Lactate	<2 mmol/L	2–4 = significant · >4 = critical / septic shock
Calcium (corrected)	2.1–2.6 mmol/L	Always correct for albumin. Formula: Ca + 0.02 × (40 − albumin)
Albumin	35–50 g/L	Low = malnutrition, liver disease, nephrotic syndrome
Bilirubin (total)	<21 µmol/L	Jaundice visible at >35 µmol/L
ALT / AST	<45 IU/L	Hepatocellular damage. Very high = hepatitis/ischaemia
ALP	30–130 IU/L	↑ in cholestasis, bone disease, pregnancy
TSH	0.4–4.5 mU/L	↓ = hyperthyroid (or pregnancy T1) · ↑ = hypothyroid
Troponin	Lab specific (hs-TnI)	Serial samples at 0 and 3h (high-sensitivity). Rise + fall = ACS.
BNP / NT-proBNP	<100 pg/mL (BNP)	Heart failure marker. Very high = decompensated HF.
D-dimer	<0.5 mg/L	Only useful if low pre-test probability. High sensitivity, low specificity.
Amylase / Lipase	<100 IU/L	>3× upper limit = pancreatitis likely. Lipase more specific.
PSA	Age-dependent	Interpret with DRE. Elevated ≠ cancer. Discuss with urology.

ABG Normal Values

Parameter	Normal Range	Abnormal → Action
pH	7.35–7.45	<7.2 = critical escalation · >7.55 = critical escalation
PaO₂	11–13 kPa	<8 kPa = type 1/2 respiratory failure
PaCO₂	4.7–6.0 kPa	>6.5 + acidosis = type 2 RF · NIV or ventilation
HCO₃⁻	22–26 mmol/L	Low = metabolic acidosis · High = metabolic alkalosis
Base excess	−2 to +2	≤−4 = significant metabolic acidosis
Lactate	<2 mmol/L	>4 = ICU escalation
SpO₂ target (general)	94–98%	COPD risk: target 88–92%

ECG Normal Values

Parameter	Normal Range	Abnormal
Heart rate	60–100 bpm	<60 = bradycardia · >100 = tachycardia
PR interval	120–200 ms	>200 ms = 1st degree heart block
QRS duration	<120 ms	≥120 ms = bundle branch block or ventricular rhythm
QTc	<440 ms (M) / <460 ms (F)	Prolonged = torsades risk (drugs, hypokalaemia)
ST segment	Isoelectric	↑ ≥1mm in ≥2 leads = STEMI · ↓ = ischaemia/NSTEMI

Paediatric Vital Signs by Age

Age	HR (bpm)	RR (/min)	SBP (mmHg)
Neonate (<1m)	120–160	30–60	60–80
Infant (1–12m)	110–150	25–50	70–100
Toddler (1–3y)	90–140	20–35	80–110
School child (4–12y)	70–110	15–25	90–120
Adolescent (12–18y)	60–100	12–20	100–130

Paediatric Red Flags:
Temperature >38°C in infant <3 months = EMERGENCY — escalate immediately
Capillary refill >3 seconds = concerning — assess circulation
Neonatal glucose <2.6 mmol/L = hypoglycaemia — treat immediately
Non-blanching rash + fever = meningococcal disease until proven otherwise

Pregnancy Lab Changes

Test	Expected Change	Clinical Note
Haemoglobin	↓ (100–120 g/L)	Physiological haemodilution — Hb 105 may be normal in pregnancy
WCC	↑ (up to 15 ×10⁹/L)	Normal in pregnancy — do not treat as infection without clinical context
Platelets	Slight ↓	Gestational thrombocytopenia is common and usually benign
Urea & Creatinine	↓	GFR increases in pregnancy — "normal" creatinine may mask impairment
ALP	↑ (placental production)	Normal in pregnancy — isolated ↑ ALP does not indicate liver disease
Albumin	↓	Must adjust corrected calcium calculation
TSH	Slight ↓ (T1)	HCG stimulates thyroid in first trimester — not always pathological
BP	↓ early, then normal	BP >140/90 after 20w = pre-eclampsia screen (+ urine protein)