When are laboratory tests indicated for children?

When to Obtain Laboratory Tests in Children

Laboratory testing in children should be guided by specific clinical indications rather than routine screening, with the threshold for testing being higher than in adults due to greater test-related burden and the need to avoid unnecessary harm. 1

General Principles for Pediatric Laboratory Testing

Pediatricians should be more restrictive and deliberate in test ordering for children compared to adults, as the perceived test-related burden (pain, anxiety, blood volume requirements) is significantly higher in pediatric populations. 1 Laboratory evaluations should be ordered only when there is a high clinical index of suspicion for a specific disease entity, as no individual test serves as an appropriate "screening" tool for most conditions. 2

Key Decision-Making Factors

When considering laboratory testing in children, base decisions on: 3

• Prevalence of the suspected condition in the pediatric population
• Patient and family history suggesting specific diagnoses
• Ease of testing and technical feasibility for the child's age
• Blood volume required relative to the child's size
• Specific clinical findings that increase pre-test probability

Specific Clinical Scenarios Requiring Laboratory Testing

Suspected Child Abuse with Bleeding/Bruising

Complete medical, trauma, and family histories plus thorough physical examination are critical before ordering any tests. 3 Laboratory evaluation for bleeding disorders is indicated when: 3

• Specific elements of history suggest bleeding disorder (family history, unusual bleeding patterns)
• Characteristics of bleeding/bruising are atypical for abuse alone
• Intracranial hemorrhage is present (though testing should be delayed until transfused blood products are eliminated if given) 3

Important caveat: Extensive laboratory evaluation is NOT necessary in many situations given the rarity of most bleeding disorders. 3 Testing suggestive of a bleeding disorder does not eliminate abuse from consideration. 3

Chronic Cough (>4 Weeks Duration)

For all children ≤14 years with chronic cough, obtain chest radiograph and age-appropriate spirometry (pre- and post-β2 agonist). 3, 4 These tests have infinite positive likelihood ratios (excellent for ruling in disease when abnormal) but poor negative likelihood ratios (cannot rule out disease when normal). 3

Do NOT routinely perform additional tests (skin prick test, Mantoux, bronchoscopy, chest CT) unless individualized based on specific clinical symptoms and signs. 3

Additional testing considerations: 3

• Bordetella pertussis testing when clinically suspected (contact history, post-tussive vomiting, whoop)
• Airway hyperresponsiveness testing only in children >6 years when asthma is suspected but spirometry is normal

Psychotic Symptoms

All children and adolescents with psychotic symptoms require thorough pediatric and neurological evaluation to rule out organic psychosis. 3

Basic laboratory tests to consider based on clinical presentation: 3

• Complete blood count
• Serum chemistry studies
• Thyroid function tests
• Urinalysis
• Toxicology screens
• HIV testing if risk factors present
• Chromosomal analysis if developmental syndrome features present

Neuroimaging, EEG, or neurology consultation are warranted only when evidence of neurological dysfunction is present. 3

Newborn Screening Follow-Up

When infants screen positive on newborn screening, diagnostic confirmation requires specific follow-up testing at specialty care centers rather than repeat screening tests. 3 The goal is to distinguish true positives from false positives and identify clinically asymptomatic conditions with gene polymorphisms. 3

Rheumatologic Concerns

Order specific rheumatologic tests only when there is high clinical suspicion for a particular disease entity, not as general screening. 2 Common tests include: 5, 6

• ESR and CRP for confirming diagnosis and monitoring disease activity
• Complete blood count for detecting cytopenia/anemia
• Complement levels (C3/C4) for SLE follow-up
• Autoantibodies when specific connective tissue diseases suspected
• Genetic studies for hereditary periodic fever syndromes

Synovial fluid analysis in children is primarily indicated for suspected joint infection, not routine arthritis evaluation. 6

Age-Specific Considerations

Infants <6 Months

Over-the-counter testing of medications should never drive laboratory evaluation; focus on clinical assessment. 7 Consider pertussis testing if paroxysmal cough, post-tussive vomiting, or inspiratory whoop present. 7

Preschool Children (2-6 Years)

Most pulmonary function tests require children >3 years old for feasibility, and even then, only 58-73% can perform adequately. 3 Standard spirometry quality control criteria derived from adults often cannot be met. 3

School-Age Children (>6 Years)

Peak expiratory flow measurements require proper technique with minimum 3 attempts recorded, though approximately 50% of children aged 4-8 years cannot perform adequately. 8

Common Pitfalls to Avoid

• Ordering tests to appease parents rather than based on clinical indication 1
• Using adult testing approaches in pediatric patients 4
• Performing "screening panels" without specific clinical suspicion 2
• Misinterpreting falsely prolonged aPTT (can occur with lupus anticoagulant or factor XII deficiency without true bleeding disorder) 3
• Testing too early after blood product transfusions in children with intracranial hemorrhage 3
• Failing to consider specimen handling issues that commonly lead to false-positive coagulation results 3

When Consultation is Indicated

Consult pediatric subspecialists when: 3

• Expanded testing beyond initial evaluation is necessary
• Preliminary testing suggests presence of specific disorder
• Testing for very rare conditions is preferred
• Interpretation of abnormal results is unclear