A clinical reference sheet

Labs Worth Asking For

The markers most physicians don’t routinely order — and what they tell you about insulin resistance, cardiometabolic risk, inflammation, and nutrient status before HbA1c and standard lipid panels do.

For dietitians, patients, and curious clinicians. Discuss any new test with the physician ordering it.

  1. 01

    HOMA-IR

    Homeostatic Model Assessment of Insulin Resistance
    Measures
    Insulin resistance. Calculated from fasting glucose × fasting insulin.
    Optimal
    <1.0 optimal; >2.0 concerning; >2.5 clinical IR
    Why it matters
    Detects insulin resistance years before HbA1c shifts. The earliest reliable signal.
    How to ask
    Order fasting insulin alongside fasting glucose, then calculate.
  2. 02

    Homocysteine

    Measures
    Amino acid metabolized through B-vitamin pathways (B12, folate, B6).
    Optimal
    <7 µmol/L optimal; 7–9 borderline; >9 elevated
    Why it matters
    Elevated levels indicate impaired methylation. Cardiovascular and cognitive risk marker.
    How to ask
    Pair with B12, folate, and B6 status to identify the deficient cofactor.
  3. 03

    Vitamin B12 + MMA

    Serum B12 + Methylmalonic Acid
    Measures
    Functional B12 status, not just circulating levels.
    Optimal
    Serum B12 >400 pg/mL; MMA <0.30 µmol/L
    Why it matters
    The standard "in range" (200–900) includes functional deficiency. MMA confirms.
    How to ask
    If serum B12 is <400, order MMA to rule out functional deficiency.
  4. 04

    Vitamin D (25-OH)

    25-hydroxyvitamin D
    Measures
    Storage form of vitamin D.
    Optimal
    40–60 ng/mL (some sources, 50–80)
    Why it matters
    Regulates insulin sensitivity, bone health, immune function. Below 30 warrants supplementation.
    How to ask
    Just ask. It's cheap and frequently low.
  5. 05

    HS-CRP

    High-Sensitivity C-Reactive Protein
    Measures
    Low-grade systemic inflammation.
    Optimal
    <0.5 mg/L low risk; 1–3 average; >3 high
    Why it matters
    Cardiovascular risk stratification beyond LDL. Useful for tracking inflammatory load.
    How to ask
    Pair with ApoB for a real cardiometabolic risk picture.
  6. 06

    ApoB

    Apolipoprotein B
    Measures
    Count of atherogenic particles.
    Optimal
    <80 mg/dL low risk; <60 for established CVD
    Why it matters
    Better than LDL-C for actual cardiovascular risk. Increasingly the standard of care.
    How to ask
    ApoB tells you what LDL particle number really means.
  7. 07

    Fructosamine

    Measures
    Glycated serum proteins — 2-3 week glucose average.
    Optimal
    175–280 µmol/L (lab-dependent)
    Why it matters
    Much faster feedback than HbA1c when adjusting medications, diet, or behavior.
    How to ask
    If you're iterating on a protocol and don't want to wait three months for the A1C.
  8. 08

    RBC Magnesium

    Red Blood Cell Magnesium
    Measures
    Cellular magnesium status — not serum.
    Optimal
    5.0–6.5 mg/dL (RBC magnesium)
    Why it matters
    Serum magnesium is poorly correlated with cellular status. RBC is the real measure.
    How to ask
    Specifically request RBC magnesium. Serum alone is misleading.
  9. 09

    Ferritin

    Measures
    Iron stores.
    Optimal
    50–150 ng/mL (women); 100–200 (men)
    Why it matters
    Low ferritin signals iron deficiency well before anemia or low hemoglobin appears.
    How to ask
    If low, evaluate menstrual blood loss, GI bleeding, or absorption.
  10. 10

    Full Thyroid Panel

    TSH + free T4 + free T3 + rT3 + antibodies
    Measures
    Comprehensive thyroid function, beyond TSH alone.
    Optimal
    TSH 1.0–2.0; fT3 upper third of range; rT3 <25% of fT3
    Why it matters
    TSH alone misses peripheral T4→T3 conversion issues and autoimmune thyroid disease.
    How to ask
    TSH + fT4 + fT3 + TPO/TgAb antibodies. Add reverse T3 if symptomatic with normal TSH.