At a glance
What this section is really testing
- Spotting the abnormality that matters among several printed results, some of which are distractors.
- Judging significance: mildly abnormal and stable is different from abnormal and moving.
- Linking the abnormality to a medicine, either as cause or as the drug whose dose now depends on it.
- Knowing the action threshold: the value at which the BNF or the treatment summary says continue, reduce, hold or stop.
A repeatable approach
- Spot the abnormalityRead every result against its range. Note direction and, where given, the trend.
- Decide whether it is significantDoes it cross a threshold that changes management, or is it noise?
- Link it to a medicineWhich drug could have caused it, and which drug's dosing depends on it?
- Find the action thresholdMonitoring requirements or renal impairment sections state when to act.
- Continue, adjust, hold or stopChoose the action, and only the action, the threshold calls for.
Practise data interpretation questions →
BNF strategy for this section
The exam is open book, and the time is short. Know which heading answers which question before you open the monograph.
The general technique, with drills, is in the BNF speed guide.
One pattern for every result
The results the exam tends to print: renal function and electrolytes, drug concentrations, liver enzymes, blood counts and observations. Each links to a small set of drugs whose dosing depends on it, and the BNF states the threshold under monitoring requirements or renal impairment.
The options are usually a graded set: continue, reduce, withhold, stop. Two of them are close. Read the threshold the BNF gives and choose the action it names, not the more cautious one you would prefer.
Common traps
Areas worth being comfortable with
Not predictions. These are the broad clinical areas this section draws on, taken from the shape of the exam and the BNF; being fluent in them makes the lookup fast.
- Renal function and the drugs dosed by eGFR
- Potassium and sodium disturbances on common drugs
- Drug levels: lithium, digoxin, gentamicin, vancomycin
- Liver function on statins, antiepileptics and antibiotics
- Blood counts on immunosuppressants and clozapine
- INR on warfarin
- Glucose on steroids and insulin
- Thyroid function on amiodarone and levothyroxine
- Observations: blood pressure, heart rate, oxygen saturation
A real data interpretation question from the bank
This is one of the 20 items every visitor can try free, in the interface the exam uses. Work it before you open the answer.
An 81-year-old man is reviewed on the orthopaedic ward 3 days after a hemiarthroplasty for a fractured neck of femur. Nursing staff report that he has become increasingly drowsy overnight and needed waking for his morning observations, although he answers questions when roused. His hip pain has been well controlled. He has eaten and drunk very little since surgery.
PMH: Hypertension; osteoarthritis; benign prostatic hyperplasia.
DH: Morphine sulfate m/r 20 mg PO twice daily (12-hrly) and morphine sulfate oral solution 5 mg PO as required (both started after surgery), paracetamol 1 g PO four times daily (6-hrly), senna 15 mg PO nightly, ramipril 5 mg PO daily, amlodipine 5 mg PO daily, tamsulosin hydrochloride 400 micrograms PO daily. No known drug allergies.
SH: Lives alone; independent before the fall; lifelong non-smoker. His ramipril has been withheld since yesterday and intravenous fluids have been started.
On examination: Temperature 36.6°C, HR 84/min and rhythm regular, BP 112/68 mmHg, RR 11/min, O₂ sat 95% breathing air. Drowsy but rousable to voice; pupils small but reactive; occasional myoclonic jerks of both hands. Chest clear. Wound clean and dry. Abdomen soft; bladder not palpable. The observation chart shows RR 18/min on the evening of surgery 3 days ago, 14/min yesterday evening and 11/min at 06.00 h today.
Investigations: Hb 103 g/L (130–170) WCC 9.6 ×10⁹/L (3.0–10.0) Platelets 318 ×10⁹/L (150–400) Na⁺ 141 mmol/L (137–144) K⁺ 5.1 mmol/L (3.5–5.3) U 5.6 mmol/L (2.5–7.0) on admission; 15.8 today. Cr 88 µmol/L (60–110) on admission 3 days ago; 141 yesterday; 196 today. eGFR 74 mL/min/1.73 m² (>60) on admission; 29 today. CRP 58 mg/L (<5)
- apply a fentanyl transdermal patch and stop morphine
- continue morphine at the current dose with hourly observations
- give naloxone immediately and stop all opioid analgesia
- halve the dose of morphine and continue at the same frequency
- stop morphine and prescribe oxycodone when the next dose of analgesia is due
Answer and marking
stop morphine and prescribe oxycodone when the next dose of analgesia is due
Marks2
ExplanationCorrect answer: stop morphine and prescribe oxycodone when the next dose of analgesia is due. The serial creatinine (88 → 141 → 196 µmol/L) shows a worsening AKI, and the falling respiratory rate (18 → 14 → 11/min) with drowsiness, small pupils and myoclonus is early toxicity from accumulating morphine-6-glucuronide. Oxycodone is predominantly hepatically metabolised and, at a reduced dose, is the safe titratable replacement; a fentanyl patch is right in principle but too slow and untitratable for acute pain, and naloxone is not yet indicated in a rousable patient with maintained oxygen saturation.
Try another data interpretation question →
More free data interpretation questions, each with its full explanation
Before you submit
- Have I found the result tied to a medicine, not just an abnormal one?
- Is it significant, or noise?
- Which drug does this threshold apply to?
- Is my action the one the threshold calls for, no more and no less?
- Would a second result confirm a trend before acting?
Practice questions
86 Data Interpretation questions, easy to hard, marked like the exam
Every answer comes with the explanation and the BNF reference behind it. Twenty questions across all eight sections are free. Pass guarantee: fail after 400+ questions and your access is extended free for 3 months.
Continue revising
View all 8 sections and the exam overview →
Section weightings follow the published PSA blueprint. The BNF headings named here are the current online BNF's. Check any dose or threshold against the BNF before you rely on it; this page teaches the method, the BNF holds the numbers.