Ordering relevant investigations and making an appropriate paediatric diagnosis
Paediatric diagnosis begins with stabilisation, history and examination. Tests should answer a specific question, change management or identify a dangerous alternative. Children have smaller blood volume, age-dependent reference ranges and different pre-test probabilities; indiscriminate testing causes iatrogenic anaemia, false positives, cost and delayed treatment.
Order the smallest set of tests that safely distinguishes urgent diagnoses and guides treatment; never let a test delay resuscitation.
Learning objectives
- Formulate a problem representation and prioritised differential before ordering tests.
- Select age-appropriate laboratory, microbiological and imaging investigations.
- Interpret results using age, gestation, nutrition, treatment and local disease prevalence.
- Recognise when bedside tests, referral or empiric treatment are more appropriate than extensive testing.
- Communicate results, uncertainty, follow-up and safety-netting to families.
Diagnostic reasoning sequence
- Stabilise: airway, breathing, circulation, disability, exposure, glucose, oxygen and urgent treatment.
- Define the syndrome: respiratory distress, fever, diarrhoea/dehydration, seizure, jaundice, anaemia, malnutrition, rash or developmental concern.
- Estimate pre-test probability: age, exposure, immunisation, malaria/TB/HIV prevalence, season and comorbidity.
- Choose a focused test: ask what result would change management.
- Interpret in context: compare with age-specific ranges and clinical trajectory.
- Act and reassess: treat, observe, refer or repeat testing only when justified.
Core investigations
| Test | Common questions answered | Important cautions |
|---|---|---|
| Point-of-care glucose | Hypoglycaemia in altered, shocked, malnourished or very young child | Treat severe low glucose immediately; confirm unexpected values |
| Pulse oximetry | Hypoxaemia and need for oxygen/referral | Check probe, perfusion, motion and waveform |
| Full blood count | Anaemia, leukocytosis/leukopenia, platelets | Age-specific ranges; malaria, HIV and malnutrition alter results |
| Blood film/RDT | Malaria parasites where endemic; morphology | Negative test does not exclude other causes of fever |
| Urinalysis/culture | UTI, renal disease, dehydration, diabetes | Collection method strongly affects contamination |
| Electrolytes/renal/liver tests | Dehydration, renal injury, jaundice, drug safety | Interpret with fluid status and age |
| Blood culture | Sepsis/meningitis pathogen before antibiotics | Do not delay antibiotics in a critically ill child |
Investigating common syndromes
Fever
Assess danger signs and local malaria risk. Test for malaria where indicated, obtain urine testing in infants with unexplained fever, and take blood cultures for suspected sepsis. Consider HIV, TB, typhoid, meningitis, pneumonia, inflammatory disease and malignancy according to history and examination. Avoid broad “fever panels” without a clinical question.
Cough or difficult breathing
Pulse oximetry is central. Chest radiograph is reserved for severe, atypical, persistent or complicated disease, suspected foreign body, heart failure or tuberculosis. Viral PCR may be useful in selected outbreaks or isolation decisions but does not replace clinical assessment. Blood tests are not routinely required for uncomplicated bronchiolitis.
Diarrhoea and vomiting
Clinical dehydration classification comes first. Check glucose and electrolytes in severe dehydration, shock, prolonged vomiting, renal disease or altered consciousness. Stool microscopy/culture is indicated for blood, suspected cholera, persistent disease, outbreak investigation or immunocompromise. Test for parasites according to duration and local epidemiology.
Seizure or altered consciousness
Check glucose immediately, assess malaria risk and meningitis signs, and consider electrolytes, calcium, toxicology and neuroimaging based on context. Lumbar puncture is urgent when meningitis is suspected after stabilisation and appropriate imaging precautions; never delay empiric antibiotics in a child with suspected bacterial meningitis.
Malnutrition
Measure weight, length/height, MUAC and oedema. Investigate glucose, haemoglobin, infection and electrolytes in complicated severe acute malnutrition. Chest radiograph, HIV/TB tests, stool or endocrine studies are targeted to clinical clues, not routine in every child.
Imaging and procedures
- Ultrasound: no ionising radiation; useful for abdomen, kidneys, pylorus, collections, heart and some lung disease.
- Radiographs: use justified views and shielding; interpret with age and clinical findings.
- CT/MRI: reserve for urgent structural, neurological or complicated disease; weigh radiation, sedation and transfer risks.
- Lumbar puncture: obtain after stabilisation when meningitis/encephalitis is suspected; check contraindications and measure opening pressure when relevant.
- Blood sampling: minimise volume, use paediatric tubes and document total blood taken, particularly in neonates and chronically ill children.
Reference ranges and test performance
Use the laboratory’s age-, sex- and method-specific reference ranges. Gestational age, prematurity, altitude, dehydration, recent transfusion, nutrition and medicines can shift results. Sensitivity, specificity, positive predictive value and negative predictive value depend on prevalence. A normal test may not exclude disease early; a weakly positive test may be false positive when pre-test probability is low.
Making the diagnosis
State the level of certainty: confirmed, probable, possible or ruled out. Separate syndrome from cause (for example, “severe pneumonia with hypoxaemia, likely bacterial”); list immediate threats, comorbidities and social risks. Include severity, complications and response to treatment. When evidence is insufficient, document a working diagnosis and a clear reassessment plan.
Communicating results and safety-netting
- Explain what was tested, what it means, what remains uncertain and what happens next.
- Give written or demonstrated medication instructions and return precautions.
- Tell caregivers to return urgently for worsening breathing, inability to drink, repeated vomiting, convulsion, lethargy, cyanosis, reduced urine, persistent fever or new rash.
- Arrange review of pending cultures, imaging and pathology; never assume the family will be contacted automatically.
OSCE checklist
- Stabilises first and checks glucose/oxygen when indicated.
- Forms a differential before ordering tests.
- Chooses age-appropriate, question-driven investigations.
- Uses local prevalence and age-specific reference ranges.
- Explains consent, pain reduction, risks, results and follow-up.
References
- SlideShare: Common laboratory investigations in paediatric practice.
- WHO IMCI and Pocket Book of Hospital Care for Children.
- WHO child growth standards and Uganda Clinical Guidelines.
Safety note: Follow current local protocols for test availability, referral, antibiotic timing, blood-volume limits and interpretation.
