Reminder
Do not order 20 investigations
before thinking.
The sequence
History and examination come first. Investigations support clinical
reasoning — they do not replace it. A test ordered before thinking
is a test without a purpose.
The standard
Good clinicians do not order many tests. They order the right tests
— chosen because they answer a specific, pre-stated clinical
question.
The obligation
If you cannot state the question the test is answering, you should
not order the test.
Core Principle
"An investigation must answer a question."
State the question before you order the test. If you cannot — rethink.
Purposes
Why do we investigate? — two tiers, five purposes
Diagnostic Tier
Used to establish or rule out a diagnosis
You already suspect the disease. The test verifies it.
Examples
Suspected ALL → FBC + peripheral smear.
Suspected nephrotic syndrome → urine protein.
You are not proving your main diagnosis — you are ruling out a
dangerous alternative.
Example
Child with wheeze + unilateral reduced air entry → chest X-ray
to exclude foreign body aspiration.
Management Tier
Used to guide decisions after the diagnosis is established
The diagnosis is known. You are assessing severity or disease
consequences.
Examples
Nephrotic syndrome → renal function + lipid profile.
Severe asthma → blood gases.
Treatment is running. You are checking whether the patient is
responding.
Examples
TB → clinical and radiological response.
Leukaemia → blood counts during chemotherapy.
The result will directly change what you do next.
Examples
Before chemotherapy → baseline renal + liver function.
Before surgery → coagulation profile if indicated.
Approach
Two ways to use investigations
— both are valid, both require logic
Testing Approach
Hypothesis-Driven vs. Exploratory Testing
Approach A — Hypothesis-Driven
You expect a specific result
You suspect a disease. You order a test expecting a particular
result. The result either
strengthens or challenges your hypothesis. This
is scientific clinical reasoning.
Example
Suspected iron deficiency → expect
low ferritin, low MCV, low MCH.
Result matches → diagnosis strengthened.
Result does not match → rethink the hypothesis.
Approach B — Exploratory
You are gathering information
Sometimes you genuinely do not know. The presentation is
undifferentiated. You order baseline investigations to
narrow the field. This is legitimate — but must
still be logical, not random.
Example
Unexplained fever of unknown origin → FBC, CRP,
blood culture, urinalysis.
Each test has a reason — together they map the territory.
Both approaches are valid. The difference is not in
the tests ordered — it is in whether the clinician has a reason for
each test. Exploratory testing that is logical and purposeful is good
medicine. Random testing — even if it accidentally finds something —
is not.
New Addition — A Critical Skill
What if the result surprises you?
When a result does not match your expectation, it is one of the most
important moments in clinical reasoning. Do not dismiss it — and do
not panic. Work through it systematically.
1
Check the sample first. Was it labelled
correctly? Was there haemolysis or a processing error? A result
that makes no clinical sense may be a laboratory issue.
2
Reconsider the diagnosis. Could the unexpected
result be telling you something true? Is there a disease you have
not yet considered that this result would fit?
3
Do not dismiss unexpected findings. Incidental
findings and unexpected results have led to important diagnoses.
Treat them as new information — not as noise to ignore.
4
Repeat if necessary. A single unexpected result
on a test prone to variability is worth repeating before acting on
it. Clinical decisions should rarely rest on one anomalous result
alone.
Safety
Before ordering — is it safe?
Contraindications
Not every investigation is safe for every patient
CT Scan
Radiation exposure
Particularly relevant in paediatrics where
cumulative radiation risk is higher. Consider whether a
non-ionising alternative (ultrasound, MRI) answers the same
question.
Contrast Studies
Renal impairment risk
Iodinated contrast can cause
contrast-induced nephropathy. Check renal
function before ordering. Hydration and dose minimisation are
important precautions.
Lumbar Puncture
Raised intracranial pressure
LP in the presence of raised ICP risks tonsillar
herniation. Always assess for papilloedema and focal neurology
before proceeding. CT head first if in doubt.
Checklist
Before You Order — five questions, every time
The Pre-Investigation Checklist
Five Questions Before Every Test
Apply these before ordering any investigation — at the bedside, on the
ward, in the exam.
01
What am I looking for?
State the specific question this test will answer. If you cannot,
do not order it.
02
Is it indicated?
Does the clinical picture justify this test? Does it serve one of
the five purposes?
03
Is it safe?
Are there contraindications for this patient? Radiation, renal
function, ICP?
04
What if positive?
What will you do if the result confirms the diagnosis or concern?
05
What if negative?
Will a negative result reassure you or change your management? If
not — why order it?
If the result will not change what you do — you should not order
the test.
This is the most powerful filter. Tests that are ordered "just in
case" or "for completeness" fail this criterion every time.
Sequence
Investigations follow reasoning — not the other way around
Step 1
History
Step 2
Examination
Step 3
Hypothesis
Step 4
Investigation
Step 5
Reassessment
Investigation sits in the middle of the clinical process — not at the
beginning.
History and examination generate the hypothesis. The investigation
tests it. Reassessment interprets the result.
This sequence connects directly to earlier frameworks in this series.
Note 3 (Predict–Treat–Reassess) covers how reassessment follows
treatment. Note 6 (Visit–Revisit) covers how history and examination
inform each other iteratively before a hypothesis is formed.
Investigations are the bridge between the hypothesis and the
management decision.
Common Student Errors
What poor investigative thinking looks like
Ordering tests before forming a clinical hypothesis
Ordering everything "just in case" — without a stated purpose for
each
Not knowing what result to expect before the test is run
Not knowing what to do when an abnormal result returns
Ignoring contraindications to investigation
Dismissing an unexpected result rather than reconsidering the
diagnosis
Final Take-Home Message
"Investigations are tools.
Not decorations. Not routines.
Not
replacements for thinking."
Good clinicians do not order many tests.
They order the right
tests — at the right time — for the right reason.
Think first
Test second
Act responsibly