Evidence-based · Longevity
The 9 Blood Markers Behind Your Biological Age
The nine PhenoAge blood markers behind your biological age, what each one signals, typical ranges, and why albumin and lymphocytes push the score down.
Part ofThe Longevity Guide→If a biological age calculator has ever given you a “phenotypic age,” it did so using just nine blood markers plus your age. Not a special panel: every one of them shows up on a routine complete blood count (CBC) and comprehensive metabolic panel that most people already get at a physical. This article walks through all nine: what each measures, roughly where “normal” sits, and which direction it pushes your biological age.
This is educational information, not medical advice. Reference ranges below are general adult figures and vary by lab, age, and sex; your own results should be interpreted by a clinician against your lab’s ranges and your history.
Where the nine came from
The nine markers weren’t chosen for elegance. In the 2018 PLOS Medicine study by Liu, Levine, and colleagues, researchers screened many candidate lab values against actual mortality data and kept the combination that best predicted who would die over the following years. The same nine feed the epigenetic DNAm PhenoAge described in the companion Aging paper. Together they span four body systems, which is exactly why the composite captures aging better than any single test.
Seven of the nine make you “older” as they rise. Only two, albumin and lymphocyte percentage, make you “younger” as they rise. Keep that split in mind; it’s the quickest way to read your own panel.
Blood PhenoAge runs on nine standard lab markers plus your age; all of them appear on a routine CBC and metabolic panel.
| Marker | System | Typical adult range | Direction in PhenoAge |
|---|---|---|---|
| Albumin | Liver / nutrition | ~3.5–5.0 g/dL | Higher = younger |
| Creatinine | Kidney | ~0.6–1.3 mg/dL | Higher = older |
| Glucose (fasting) | Metabolic | ~70–99 mg/dL | Higher = older |
| C-reactive protein | Inflammation | < ~3 mg/L | Higher = older |
| Lymphocyte % | Immune | ~20–40% | Higher = younger |
| Mean cell volume (MCV) | Red cells | ~80–100 fL | Higher = older |
| Red cell distribution width (RDW) | Red cells | ~11.5–14.5% | Higher = older |
| Alkaline phosphatase (ALP) | Liver / bone | ~44–147 U/L | Higher = older |
| White blood cell count (WBC) | Immune | ~4.0–11.0 ×10⁹/L | Higher = older |
(The original models use SI units, for example albumin in g/L, creatinine in µmol/L, glucose in mmol/L, but the direction of each effect is the same regardless of the units your report uses.)
The inflammation and immune markers
C-reactive protein (CRP). A liver-made protein that rises with inflammation. Chronic low-grade inflammation, sometimes called “inflammaging,” is one of the clearest drivers of an accelerated score. PhenoAge uses the log of CRP, so it’s most sensitive at the low end. Because CRP spikes with any acute infection or injury, it’s the marker most likely to distort a single test. We cover it in depth in inflammation markers, hs-CRP, and aging.
White blood cell count (WBC). Total immune cells in circulation. A higher count can reflect ongoing inflammation or immune activation; within the normal range, lower tends to read as biologically younger.
Lymphocyte percentage. The share of white cells that are lymphocytes (T and B cells). A higher percentage signals a stronger adaptive immune profile, one of only two markers where up is good. It tends to drift down with age as immune function shifts.
The red-cell markers
Mean cell volume (MCV). The average size of your red blood cells. It creeps upward with age and with things like B12/folate status and alcohol intake; larger average cell size reads as older.
Red cell distribution width (RDW). How variable your red cells are in size. This unassuming CBC line is one of the strongest single predictors in the whole panel: greater variability consistently associates with higher mortality and aging risk, even when the value is technically “normal.” Higher RDW pushes phenotypic age up.
The metabolic marker
Glucose. Fasting blood sugar. Higher glucose reflects insulin resistance and metabolic stress, both tightly linked to aging outcomes, so it pushes the score older. This is one of the most modifiable inputs on the panel through diet, activity, and body composition, which is a good reason to make sure the sample was genuinely fasting.
The liver and kidney markers
Albumin. The main protein in blood, made by the liver. It reflects nutritional status, liver function, and overall physiological reserve. Higher albumin is protective, the single strongest “younger” signal, and low albumin is a well-documented flag for frailty and mortality risk in older adults.
Alkaline phosphatase (ALP). An enzyme from liver and bone. Elevated ALP can reflect liver or bone turnover issues; higher values read as older.
Creatinine. A muscle-breakdown byproduct cleared by the kidneys. Higher creatinine usually signals reduced kidney filtration, so it raises phenotypic age. One caveat: very muscular people can run higher creatinine for benign reasons, which is a reminder that no single marker should be read in isolation.
How the markers combine
Each marker is multiplied by a weight from the mortality model, summed with your chronological age, and converted into an age in years. That’s why two people with identical “all normal” panels can still get different phenotypic ages. PhenoAge doesn’t just ask is this in range?, it asks how far is this from where a younger person’s value would typically sit? A cluster of values drifting toward the “older” end of normal can nudge your score up even with nothing flagged out of range.
It also means no single marker makes or breaks your result. A one-off elevated CRP from a cold won’t wreck a strong overall profile, and one great value can’t rescue an otherwise poor panel. Read the whole set together.
Using this in practice
- Get the standard panels done fasting and healthy. A CBC plus a comprehensive metabolic panel with CRP covers all nine.
- Look at the seven “older” markers first (glucose, CRP, RDW, MCV, WBC, creatinine, ALP), since those are usually where an elevated score comes from.
- Check the two “younger” markers, albumin and lymphocyte percentage, as reserve indicators.
- Track trends, not single readings. Because several markers are noisy, the direction over repeated tests is far more meaningful than one snapshot.
For where these nine sit among the wider field of aging measurements, and which are worth your attention, see our overview of biomarkers of aging worth tracking.
The takeaway
Your biological age score isn’t magic; it’s nine ordinary lab numbers, spanning inflammation, immunity, red cells, metabolism, the liver, and the kidney, weighted by how strongly each predicts mortality risk. Learn which way each one points (only albumin and lymphocytes are “up is good”), and the output stops being a black box. Run your latest results through the biological age calculator to see how the nine combine for you, and take anything out of range to a clinician.
References
- Liu Z, Kuo P-L, Horvath S, Crimmins E, Ferrucci L, Levine M. A new aging measure captures morbidity and mortality risk across diverse subpopulations from NHANES IV. PLoS Med. 2018;15(12):e1002718
- Levine ME, Lu AT, Quach A, et al. An epigenetic biomarker of aging for lifespan and healthspan. Aging (Albany NY). 2018;10(4):573–591 (full text)
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