Haematology
Haemoglobin
♂ 130–170 g/L · ♀ 120–155 g/L
Critical: <70 or >200 g/L
Oxygen-carrying capacity. Low = anaemia; high = polycythaemia.
↑ Causes
- • Polycythaemia vera
- • Chronic hypoxia (COPD, OSA, altitude)
- • Dehydration
↓ Causes
- • Iron / B12 / folate deficiency
- • Chronic disease
- • Bleeding, haemolysis
- • Marrow failure
WCC (total)
4–11 × 10⁹/L
Critical: <1 or >30
Total leucocytes; check differential.
↑ Causes
- • Infection
- • Inflammation
- • Steroids
- • Leukaemia
↓ Causes
- • Viral
- • Sepsis (severe)
- • Chemo
- • Marrow failure
Platelets
150–400 × 10⁹/L
Critical: <20 or >1000
Low → bleeding risk; high → thrombosis.
↑ Causes
- • Reactive (infection, iron def)
- • MPN (ET, PV)
↓ Causes
- • ITP
- • DIC
- • TTP/HUS
- • Drugs (heparin → HIT)
- • Marrow failure
Coagulation
INR
0.8–1.2 (target 2–3 on warfarin)
Critical: >5 (with bleed)
Extrinsic pathway. Used to monitor warfarin.
↑ Causes
- • Warfarin
- • Liver disease
- • Vit K deficiency
- • DIC
- • DOAC interference
APTT
26–38 s
Intrinsic pathway. Monitors heparin.
↑ Causes
- • Heparin
- • Haemophilia
- • vWD
- • Antiphospholipid (paradoxical)
- • DIC
D-dimer
<500 ng/mL (age-adjusted)
Use only with low/intermediate pre-test probability for VTE rule-out.
↑ Causes
- • VTE
- • DIC
- • Pregnancy
- • Malignancy
- • Infection
- • Recent surgery
Electrolytes
Sodium
135–145 mmol/L
Critical: <125 or >155
Assess with volume status before treating.
↑ Causes
- • Dehydration
- • DI
- • Hypertonic saline
↓ Causes
- • SIADH
- • Diuretics
- • HF / cirrhosis
- • Adrenal insufficiency
Potassium
3.5–5.0 mmol/L
Critical: <2.5 or >6.5
ECG changes guide urgency.
↑ Causes
- • AKI/CKD
- • ACEi/ARB/spironolactone
- • Tissue breakdown
- • Adrenal insufficiency
↓ Causes
- • Diuretics
- • Vomiting/diarrhoea
- • Insulin / β-agonist
- • Cushing's
Renal
Urea
2.5–7.8 mmol/L
Rises in dehydration, UGIB, AKI, high-protein diet.
↑ Causes
- • AKI/CKD
- • Dehydration
- • UGIB
- • Catabolic state
↓ Causes
- • Liver failure
- • Pregnancy
- • Low protein
Creatinine
♂ 60–110 · ♀ 45–90 µmol/L
Use to estimate GFR (CKD-EPI). Trends matter more than absolute value.
↑ Causes
- • AKI/CKD
- • Muscle injury
- • Drugs (NSAID, ACEi)
↓ Causes
- • Low muscle mass
- • Pregnancy
Bone
Calcium (corrected)
2.20–2.60 mmol/L
Critical: <1.8 or >3.5
Adjust for albumin: Ca + 0.02 × (40 − albumin).
↑ Causes
- • Primary hyperparathyroidism
- • Malignancy (PTHrP, bone mets)
- • Vitamin D toxicity
↓ Causes
- • Hypoparathyroidism
- • CKD
- • Vit D deficiency
- • Pancreatitis
Magnesium
0.7–1.0 mmol/L
Low Mg → resistant hypokalaemia/hypocalcaemia + arrhythmia.
↑ Causes
- • Renal failure
- • Mg supplementation
↓ Causes
- • Diuretics
- • PPI
- • Diarrhoea
- • Alcohol
Phosphate
0.8–1.5 mmol/L
Low → muscle weakness, respiratory failure.
↑ Causes
- • CKD
- • Hypoparathyroidism
- • Tumour lysis
↓ Causes
- • Refeeding
- • DKA recovery
- • Phosphate binders
Liver
ALT
<40 U/L
Hepatocellular pattern when >2× ALP rise.
↑ Causes
- • Viral hepatitis
- • Drug-induced (paracetamol)
- • Ischaemic hepatitis
- • NAFLD/NASH
ALP
30–130 U/L
Cholestatic / bone source. Check GGT to confirm liver origin.
↑ Causes
- • Biliary obstruction
- • Bone disease (Paget's, mets)
- • Pregnancy
- • Drugs
Bilirubin
<21 µmol/L
Split into conjugated vs unconjugated for differential.
↑ Causes
- • Haemolysis (unconj)
- • Gilbert's
- • Hepatitis
- • Obstruction (conj)
Albumin
35–50 g/L
Marker of synthetic function + nutrition; falls in inflammation.
↓ Causes
- • Liver failure
- • Nephrotic syndrome
- • Malnutrition
- • Sepsis / inflammation
Inflammatory
CRP
<5 mg/L
Acute phase reactant; >100 suggests bacterial infection.
↑ Causes
- • Infection
- • Inflammation
- • Malignancy
- • Trauma
Cardiac
Troponin (hs)
Assay-specific, often <14 ng/L
Rise + fall + symptoms = MI. Many non-ischaemic causes.
↑ Causes
- • ACS
- • PE
- • Sepsis
- • Heart failure
- • Renal failure
- • Myocarditis
Metabolic
Lactate
<2 mmol/L
Critical: >4 mmol/L
Marker of tissue hypoperfusion or impaired clearance.
↑ Causes
- • Sepsis / shock
- • Ischaemia
- • Metformin
- • Liver failure
Glucose (fasting)
3.9–5.5 mmol/L
Critical: <3.0 or >25
Diabetes if fasting ≥7.0 or random ≥11.1 with symptoms.
↑ Causes
- • Diabetes
- • Stress
- • Steroids
↓ Causes
- • Insulin / sulphonylurea
- • Sepsis
- • Adrenal insufficiency
- • Insulinoma
HbA1c
<42 mmol/mol (<6.0%)
Diabetes ≥48 mmol/mol; pre-diabetes 42–47.
↑ Causes
- • Diabetes
↓ Causes
- • Haemolysis
- • Recent transfusion
- • Pregnancy
- • Iron deficiency (falsely high)
Endocrine
TSH
0.4–4.5 mU/L
Most sensitive marker of thyroid status (except secondary).
↑ Causes
- • Primary hypothyroidism
- • Recovery from non-thyroidal illness
↓ Causes
- • Thyrotoxicosis
- • Pituitary disease
- • Steroids / dopamine
ABG
pH
7.35–7.45
Critical: <7.20 or >7.60
Acidaemia vs alkalaemia.
PaCO₂
4.7–6.0 kPa (35–45 mmHg)
Respiratory component. High = respiratory acidosis.
HCO₃⁻
22–26 mmol/L
Metabolic component. Low = metabolic acidosis.
PaO₂
>10.6 kPa (80 mmHg) on air
Critical: <8 kPa
Hypoxaemia threshold = <8 kPa.
Base excess
−2 to +2 mmol/L
Metabolic acid–base derangement; <−2 acidosis, >+2 alkalosis.
Anion gap
8–16 mmol/L
Raised AG: MUDPILES (methanol, uraemia, DKA, paraldehyde, INH/iron, lactate, ethylene glycol, salicylates).
