What serum biomarkers (glial fibrillary acidic protein [GFAP], ubiquitin‑C‑terminal hydrolase‑L1 [UCH‑L1], S100B, neuron‑specific enolase [NSE], neurofilament light chain, total tau) are recommended for acute brain injury assessment, and how should their results guide imaging and follow‑up?

Biomarkers of Brain Injury

In mild traumatic brain injury (mTBI), serum S100B measured within 4 hours of injury at levels <0.1 μg/L can be used to avoid CT scanning in patients without significant extracranial injuries, though the combination of GFAP and UCH-L1 demonstrates superior diagnostic performance with 100% sensitivity for excluding intracranial injury. 1, 2

Clinical Context and Biomarker Selection

The evidence strongly supports a tiered approach to biomarker utilization based on injury severity and timing:

For Mild TBI (GCS 13-15)

S100B remains the only biomarker with formal guideline-level recommendation, though this comes with important caveats 1. The 2008 ACEP guidelines provide a Level C recommendation that in mild TBI patients without significant extracranial injuries and serum S100B <0.1 μg/L measured within 4 hours of injury, consideration can be given to not performing CT. However, this test has not received FDA approval for clinical use in the United States 1.

The evidence landscape has evolved significantly since 2008. Recent high-quality studies demonstrate that GFAP and UCH-L1 combined outperform S100B with pooled sensitivity of 100% (95% CI 99-100%) and specificity of 31% (95% CI 26-36%), compared to S100B's sensitivity of 92% (95% CI 89-94%) and specificity of 31% (95% CI 27-35%) 2, 3. This translates to a potential 31% reduction in CT scans while maintaining perfect sensitivity for excluding intracranial injury 2.

Specific Biomarker Performance

GFAP (Glial Fibrillary Acidic Protein):

• Best single biomarker performance with AUC 0.86-0.88 4, 5, 3
• Sensitivity 89-94%, specificity 37-40% 4, 3
• Superior to S100B when sampled within 6 hours 5
• Maintains performance beyond 6 hours post-injury 5

UCH-L1 (Ubiquitin C-Terminal Hydrolase-L1):

• Lower standalone performance: sensitivity 72-83%, specificity 51-53% 5, 3
• Improved performance when sampled within 6 hours 5
• Most valuable when combined with GFAP 2

S100B:

• Most extensively studied with 90-100% sensitivity at low cutoffs (0.1 μg/L) 1
• Major limitation: very low specificity (4-65%) 1
• Half-life approximately 97 minutes, requiring early measurement 1
• Critical pitfall: Performance degrades significantly in patients with extracranial injuries due to presence in adipose tissue, skin, and cartilage 1
• Alcohol intoxication does not affect test characteristics 1

NSE (Neuron-Specific Enolase):

• Limited evidence for acute mTBI assessment 1
• Not significantly elevated in mTBI patients with CT abnormalities 4
• More relevant for severe TBI and post-cardiac arrest prognostication 6

Neurofilament Light Chain (NF-L):

• Emerging biomarker with excellent performance in post-cardiac arrest settings (AUC 0.98) 6
• Moderate performance in acute TBI (AUC 0.81 at 72 hours) 4
• Major limitation: No widely available commercial assays for clinical use 6

Total Tau:

• Moderate diagnostic performance for CT abnormalities 4
• More extensively studied in severe TBI and post-cardiac arrest prognostication 6
• Limited acute care utility in mTBI

Clinical Implementation Algorithm

Step 1: Initial Assessment

• GCS score 13-15 within 12 hours of injury
• Assess for significant extracranial injuries (fractures, burns, soft tissue trauma)
• Document time from injury to potential biomarker sampling

Step 2: Biomarker Selection Based on Available Resources

If GFAP/UCH-L1 combination available (preferred):

• Sample within 12 hours of injury (optimal within 6 hours)
• Negative result (below validated threshold): Can safely avoid CT with 100% sensitivity 2, 7, 8
• Positive result: Proceed with CT imaging
• Expected CT reduction: 28-31% 2, 3

If only S100B available:

• Sample within 4 hours of injury (critical due to 97-minute half-life) 1
• Exclude patients with significant extracranial injuries 1
• S100B <0.1 μg/L: Consider avoiding CT 1
• S100B ≥0.1 μg/L: Proceed with CT imaging
• Expected CT reduction: Lower than GFAP/UCH-L1 due to specificity limitations

Step 3: Special Populations

Intoxicated patients (GCS 13-14):

• S100B performance unaffected by alcohol 1
• GFAP/UCH-L1 demonstrate significantly higher levels in CT-positive intoxicated patients 4
• Biomarkers particularly valuable when clinical assessment limited by intoxication

Patients on anticoagulation or with bleeding disorders:

• Insufficient data to support biomarker-guided CT avoidance 1
• Proceed with standard imaging protocols

Step 4: Timing Considerations

Within 6 hours of injury:

• Optimal window for all biomarkers 5
• S100B and UCH-L1 show best performance in this window 5

6-12 hours post-injury:

• GFAP maintains superior performance to S100B 5
• S100B performance relatively stable across time periods studied 5

Beyond 12 hours:

• Limited evidence for acute decision-making
• Consider standard clinical decision rules

Critical Pitfalls and Caveats

1. S100B extracranial contamination: Absolutely contraindicated in polytrauma patients with significant soft tissue injuries, burns, or fractures 1

2. Timing is critical: S100B's 97-minute half-life means delayed sampling significantly reduces sensitivity 1

3. FDA approval status: As of the guideline evidence, S100B was not FDA-approved for clinical use in the United States 1. Verify current regulatory status before implementation.

4. Biomarkers identify CT abnormalities, not clinical significance: A negative biomarker rules out CT-detectable injury but does not eliminate the diagnosis of concussion or need for clinical follow-up 1

5. High-risk features override biomarkers: Patients with focal neurological deficits, skull fractures, or GCS <13 require imaging regardless of biomarker results 1

For Severe TBI and Post-Cardiac Arrest Prognostication

The 2018 French guidelines explicitly recommend against using biomarkers in clinical routine for initial severity assessment of severe TBI 9. However, for post-cardiac arrest neuroprognostication, NSE at 72 hours (with very high values indicating poor prognosis) and NF-L show promise as part of multimodality assessment 6.

Follow-Up Guidance

Regardless of biomarker results, all patients discharged from the emergency department must receive education about post-concussive symptoms 1. Biomarkers do not replace clinical judgment regarding need for observation, return precautions, or outpatient follow-up.