This is a practical measurement guide, not a shopping list of random labs. Start with the highest-yield basics, then add more only when your pattern still needs better explanation.
If the story includes low drive, poor recovery, sexual-function change, or suspected androgen issues, do not stop at one total testosterone number. The high-yield set is Total Testosterone, Free Testosterone, and SHBG, usually alongside LH and FSH.
Draw testosterone labs in the morning, then interpret them together rather than as isolated values.
Cheapest, highest yield. Any GP can order these. Covers the most common reversible causes.
Fog persists after Tier 1 is unrevealing. Some require a specialist referral.
Requires clinical context. Share your symptom pattern with the specialist.
Standard lab ranges are based on avoiding disease, not optimizing function. A TSH in the upper "normal" range may leave some people symptomatic — discuss your individual target with your doctor.
TSH
Normal: 0.4-4.5 → Often optimal: lower half of range*
Ferritin
Normal: >12 → Optimal: 50-150
B12
Normal: >200 → Optimal: >500
Print this section or copy it to your phone. Say: "I've been tracking a persistent brain fog pattern and want to start with the highest-yield measurements first. Could we start here?" If a clinician wants to start smaller, ask which markers would most help strengthen or weaken the leading theory.
Mainstream First
CIRS-Style Lane
Caution
Urine mycotoxin testing, single abnormal inflammatory markers, or one failed screener do not prove that mold is the cause of your brain fog. The exposure story, response away from the environment, and competing causes still matter more than one result in isolation.
| Biomarker | Optimal Range | Why It Matters | Action |
|---|---|---|---|
| 25-OH Vitamin D | 40–60 ng/mL | Severe deficiency doubles dementia risk | Detail → |
| A1c + fasting glucose context review | Interpret with timing pattern | Focused explainer for the common mismatch where average labs look normal but the symptom timing still suggests glucose variability. | Detail → |
| AASP Sensory Profile | Validated assessment | Adult sensory processing patterns | Detail → |
| Actigraphy | 7+ days tracking | Wrist-worn sleep tracker — assesses circadian patterns | Detail → |
| Active B12 (Holotranscobalamin) | >50 pmol/L | More accurate than serum B12 for cellular deficiency | Detail → |
| AM Cortisol (8am) | Lab reference interval with timing context | Morning cortisol explainer focused on the timed 8am draw clinicians usually use for baseline adrenal-context questions. | Detail → |
| ANA (Antinuclear Antibodies) | Negative (<1:40) | Screening for autoimmune disease — positive in lupus, Sjögren's, RA | Detail → |
| Anti-dsDNA Antibodies | Negative | Specific for systemic lupus erythematosus (SLE) | Detail → |
| Anticholinergic Burden Score | 0 (none) | Score >3 associated with cognitive decline | Detail → |
| AQ-10 Screening | Score <6 | Autism spectrum screening questionnaire | Detail → |
| ASL-MRI (Arterial Spin Labeling) | Normal perfusion | Non-invasive brain blood flow measurement | Detail → |
| ASRS-v1.1 (ADHD) | 0-6 screener context | Patient-facing ADHD screener route matching the ASRS-v1.1 wording used in results cards and clinician conversations. | Detail → |
| ASRS-v1.1 Screener | Validated questionnaire | Adult ADHD Self-Report Scale — 6-question screener | Detail → |
| Bartonella IgG/IgM | Negative | Bartonella infection screening | Detail → |
| Bartonella PCR | Negative | Direct detection of Bartonella DNA | Detail → |
| Baseline Cognitive Assessment | Screening context | Patient-facing explainer for starting with a validated cognitive screen such as MoCA before deciding whether fuller neuropsychology is needed. | Detail → |
| Beighton Score | <5/9 | Joint hypermobility assessment for EDS/HSD | Detail → |
| Blood Mercury | <5 μg/L | Mercury exposure — half-life ~70-80 days. Primarily reflects recent dietary/fish exposure. | Detail → |
| Blood Sugar Assessment | Panel context | Starter metabolic workup for glucose variability, reactive hypoglycemia clues, and insulin-resistance overlap. | Detail → |
| Brain MRI | Radiology report | Structural neuroimaging used to evaluate red flags and differential neurological causes. | Detail → |
| BRIEF-A | T-score <65 | Behavior Rating Inventory of Executive Function | Detail → |
| C-Reactive Protein | <3.0 mg/L | General inflammation marker — less sensitive than hs-CRP | Detail → |
| C4a Complement | <2830 ng/mL | Inflammatory marker elevated in CIRS and Lyme | Detail → |
| CBC + CMP | Lab reference interval | Baseline panel combining complete blood count and metabolic chemistry for broad screening context. | Detail → |
| CBC with Differential | Lab reference interval | Core blood count panel used to review white cell patterns, hemoglobin, and platelet context. | Detail → |
Showing the first 25 measurements. See what to check first →
RULE-OUTS
What to Check First
A practical sequence for ruling in or ruling out common explanations.
EXPLORE
All 64 Causes
See which patterns fit, what strengthens them, and what to measure next.
Use these measurements to make your pattern more legible, not to chase certainty. Standard reference ranges tell you whether something looks overtly abnormal; more useful clinical interpretation depends on symptoms, timing, and context.
Learn how we choose useful ranges and references →
Use core tests first, then optional-later tests only when pattern evidence supports it.
Bring a concise story + top causes + test shortlist to reduce repeat visits.
Tier A · A1c + fasting glucose context review
Normal average labs do not always exclude meaningful glucose variability when symptoms are strongly timing-linked.
Tier B · ASRS-v1.1 Screener
Validated ADHD screening can support differential assessment when interpreted with clinical context.
Tier B · Blood Sugar Assessment
Average-glucose markers and insulin-resistance context are more useful when interpreted together in a pattern-based metabolic workup.
Tier B · Fasting Insulin
Fasting insulin can provide additional context for insulin resistance when interpreted with glucose markers.
Tier A · HbA1c
A1c is useful for average glucose but can miss variability and excursions.
Tier B · HbA1c + Fasting Insulin
A1c and fasting insulin together can provide broader metabolic context than a single average-glucose marker alone.
This information is for educational purposes only. Always consult with a qualified healthcare professional.
Related Causes
These causes align with high-yield testing pathways.