Laboratory Tests
ME/CFS is currently a clinical diagnosis without a pathognomonic laboratory test. Laboratory testing serves two purposes: (1) excluding alternative diagnoses and (2) identifying treatable comorbidities.
1 Standard Exclusion Panel
The following tests are recommended to exclude conditions that can mimic ME/CFS (National Institute for Health and Care Excellence 2021) (Carruthers et al. 2011):
- Complete blood count: Excludes anemia, leukemia, infection
- Comprehensive metabolic panel: Excludes renal/hepatic disease, electrolyte disorders
- Thyroid function (TSH, free T4): Excludes hypothyroidism/hyperthyroidism
- Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP): Screens for active inflammatory/autoimmune conditions
- Ferritin: Excludes iron deficiency (common comorbidity)
- Vitamin B12 and folate: Excludes deficiency-related fatigue
- Cortisol (morning or ACTH stimulation test): Excludes Addison’s disease (primary adrenal insufficiency)
- Celiac serology: Excludes celiac disease
- Urinalysis: Screens for renal disease, diabetes
Standard morning cortisol screening is designed to detect primary adrenal insufficiency (Addison’s disease), where cortisol is markedly low and ACTH is elevated. Post-viral secondary adrenal insufficiency—documented in \(\sim\) 40% of SARS-1 survivors (Leow et al. 2005) and in COVID-19 pituitary case series (Carosi et al. 2024)—presents differently: cortisol falls in the low-normal range (typically not flagged as abnormal by laboratory reference ranges), and ACTH is low or inappropriately normal rather than elevated. This pattern is invisible to morning cortisol screening alone. In any post-COVID ME/CFS patient with fatigue disproportionate to other findings, unexplained hypotension, or a clinical picture consistent with cortisol insufficiency, dynamic testing should be considered: ACTH stimulation test (most accessible) or insulin tolerance test (ITT, gold standard for central AI). See Chapter Endocrine and Metabolic Dysfunction, Section Maladaptive Chronic Inflammatory Signaling for mechanistic context.
2 Extended Panel for Clinical Characterization
Additional tests characterize disease mechanisms but are not required for diagnosis:
- Immunoglobulin levels (IgG, IgA, IgM): Identifies humoral immune deficiency
- Lymphocyte subsets (CD4, CD8, NK cells): Characterizes immune cell populations
- NK cell function assay: Quantifies cytotoxic activity (research setting)
- Autoantibody panel (\(\beta_2\)-adrenergic, muscarinic M3/M4 receptor antibodies): Identifies autoimmune subtype (specialized laboratories)
- Viral serology (EBV VCA IgG/IgM, EBNA, HHV-6 IgG): Documents prior/active herpesvirus infection
- Tryptase: Screens for mast cell activation
- Vitamin D: Common deficiency in housebound patients
- Sleep study (polysomnography): Excludes primary sleep disorders (obstructive sleep apnea, narcolepsy)
3 Experimental Biomarkers
No validated diagnostic biomarker for ME/CFS exists, but several candidates are under investigation:
- Metabolomics panels: Reduced amino acids, altered lipid profiles, elevated lactate-to-pyruvate ratios (Naviaux et al. 2016) (Germain et al. 2020)
- Cytokine panels: Disease-duration-dependent profiles (Hornig et al. 2015) (Montoya et al. 2017)
- Epigenetic markers: DNA methylation signatures at specific CpG sites Vega, Vernon, and McGowan (2021)
- Two-day CPET: While not a blood test, the day-2 decrement serves as a functional biomarker (Section Objective Tests)
ME/CFS remains a clinical diagnosis based on symptom patterns (post-exertional malaise as the cardinal feature) after exclusion of alternative explanations. No laboratory test, imaging study, or biomarker has been validated for routine diagnostic use. Patients and clinicians should be cautious of commercial tests marketed as ME/CFS diagnostics that have not undergone rigorous validation.