Objective Tests
1 Two-Day Cardiopulmonary Exercise Testing (2-Day CPET)
Two-day CPET is the gold-standard objective test for demonstrating PEM and is the only test that can objectively quantify the exercise intolerance characteristic of ME/CFS (Keller et al. 2024). However, a 2026 replication study did not find group-average Day 2 VO₂ decline (Mancini et al. 2026), though both studies concur on elevated RPE and chronotropic incompetence. The CPET decline finding is contested; elevated RPE and chronotropic incompetence are consistent findings across studies.
Purpose: Objective demonstration of post-exertional physiological impairment; documentation of reduced exercise capacity for disability evaluation; research quantification of metabolic dysfunction.
Protocol: Two maximal graded exercise tests on consecutive days, typically on a cycle ergometer:
- Day 1: Standard incremental exercise test to volitional exhaustion. Measures peak VO2, anaerobic threshold (AT/VT1), peak workload, and respiratory exchange ratio.
- Day 2: Identical protocol repeated 24 hours later.
Interpretation: In healthy controls and most other chronic diseases, day-2 performance equals or exceeds day-1 performance. In ME/CFS, day-2 performance is significantly reduced:
- Peak VO2 decreases by \(\geq 8\)% (clinically significant decline)
- Anaerobic threshold decreases, indicating earlier metabolic shift to glycolysis
- Peak workload and exercise duration decrease
Clinical significance: The day-2 decrement is objective when present and specific to ME/CFS among studied conditions—however, the group-average decline finding is contested (Keller 2024 positive vs Mancini/Natelson 2026 null (Mancini et al. 2026)). Elevated RPE and chronotropic incompetence are consistent across all CPET studies but are not disease-specific metabolic biomarkers. May provide supporting evidence for PEM in disability evaluations alongside other clinical data.
Limitations: Maximal exercise testing is contraindicated in some severely affected patients; provokes PEM lasting days to weeks; limited availability; requires specialized equipment and trained personnel. Not suitable for serial monitoring due to the PEM it induces.
2 Home-Based Assessment Protocols for Severe ME/CFS
Standard clinic-based diagnostic procedures are poorly tolerated by severe and very severe ME/CFS patients (Bell score \(\leq\) 30), with \(>\) 60% unable to complete inpatient protocols due to PEM (Fricke et al. 2026). The ACHTSAM study (Fricke et al. 2026) is the first protocol to systematically evaluate which assessments can be safely administered through home visits, using a staged approach that orders procedures from low to high burden with mandatory rest periods.
- Low-burden assessments: (\(\leq\) 5min, supine-tolerant): Resting ECG, pupillography, osteosonography (bone density), bioelectrical impedance body composition, near-infrared spectroscopy (NIRS) for muscle oxygenation, light activity sensor.
- Moderate-burden assessments: (5–15min): HRV with paced breathing, Schellong orthostatic test, handgrip dynamometry, blood sampling, salivary cortisol.
- High-burden assessments: (25–30min, mental/physical exertion): Cognitive testing (MoCA, TMT-B, SDMT), EndoPAT endothelial function (peripheral arterial tonometry), neurocognitive/psychological assessment.
- Design principles: Assessments proceed in burden order; supine procedures precede seated/standing tests; each procedure includes post-assessment perceived exertion rating; patients may pause or terminate at any time.
- Tolerability hypothesis: Non-invasive supine-position assessments are expected to achieve \(\geq\) 80% completion, while high-burden tests requiring sustained mental exertion or standing are expected to achieve \(<\) 30%.
- Status: Protocol published; results expected mid-2026.
3 Tilt Table Testing
Tilt table testing evaluates autonomic cardiovascular regulation during orthostatic stress.
Purpose: Diagnosis of orthostatic intolerance, postural orthostatic tachycardia syndrome (POTS), and neurally mediated hypotension.
Protocol: The patient rests supine for 10–20 minutes, then is passively tilted to 60–70compose for up to 45 minutes. Continuous heart rate and blood pressure monitoring throughout.
Diagnostic criteria:
- POTS: Sustained heart rate increase \(\geq 30\)bpm (or \(\geq 40\)bpm in ages 12–19) within 10 minutes of tilt, without orthostatic hypotension
- Neurally mediated hypotension: Drop in systolic BP \(\geq 20\)mmHg or diastolic BP \(\geq 10\)mmHg, often with paradoxical bradycardia
- Orthostatic hypotension: Sustained drop in systolic BP \(\geq 20\)mmHg or diastolic \(\geq 10\)mmHg within 3 minutes of tilt
ME/CFS relevance: Orthostatic intolerance is present in 50–90% of ME/CFS patients. Tilt table testing provides objective documentation and identifies the specific subtype, guiding treatment selection.
Alternative: Active standing test (10-minute stand with serial HR and BP measurement) provides a simpler office-based screen, though less standardized.
4 Autonomic Function Tests
Additional autonomic assessments complement tilt table testing.
- Heart rate variability (HRV): Analysis of beat-to-beat heart rate intervals from ECG or heart rate monitor recordings. Time-domain (SDNN, RMSSD) and frequency-domain (LF, HF, LF/HF ratio) metrics quantify sympathovagal balance. ME/CFS patients typically show reduced HRV, reduced HF power (parasympathetic), and elevated LF/HF ratio (sympathetic predominance) (Newton et al. 2007).
- Valsalva maneuver: Forced expiration against a closed glottis (40mmHg for 15 seconds). The heart rate and blood pressure response has four phases testing baroreflex integrity. Abnormal responses indicate baroreflex failure.
- Deep breathing test: Heart rate response to deep breathing at 6 breaths/minute. The expiratory–inspiratory HR difference (E:I ratio) quantifies parasympathetic function. Reduced in ME/CFS.