Which Blood Tests Change as We Age?

A result that was always normal starts drifting toward the edge of the range. Your kidney estimate slips, your thyroid number rises, your blood sugar marker creeps up. If you’ve wondered which blood tests change as we age, and whether a change means disease or just birthdays, the answer depends on the test. Some reference intervals vary by age; many clinical decision thresholds do not. Humanic Blue looks at both the population data and current guidance to sort out that distinction.

Several common markers shift with age in population studies. Measured kidney filtration tends to decline. Thyroid-stimulating hormone (TSH) readings tend to be higher, and HbA1c, a measure of average blood sugar, rises modestly even among people without diagnosed diabetes. Anemia becomes more common after 50, and older U.S. survey data left about one-third of cases unexplained. None of those patterns makes an abnormal result harmless. Current kidney and diabetes definitions retain fixed decision thresholds, while thyroid interpretation depends on symptoms, repeat testing, free thyroxine, a thyroid hormone measured in blood, and clinical context. For living longer and better, the useful question is what the change means for this person—not whether birthdays get automatic veto power.

The four markers below show why a change that is common with age still needs explanation.

1. Does kidney function (eGFR) change with age?

eGFR, the estimated glomerular filtration rate, is calculated from filtration markers—usually blood creatinine—and estimates how well the kidneys filter. Under the 2024 Kidney Disease: Improving Global Outcomes (KDIGO) guideline, an eGFR below 60 mL/min/1.73 m² (milliliters filtered per minute, adjusted to a standard body surface area) is a criterion for chronic kidney disease only when the abnormality is present for more than three months; a single low result does not show that the change has persisted.

Measured filtration tends to fall with age even without other recognized kidney damage. In a 2019 review in the Journal of the American Society of Nephrology, an international group led by Pierre Delanaye argued that a fixed threshold can blur kidney aging and kidney disease and proposed an age-adapted definition.

That proposal is not current practice. KDIGO 2024 explicitly retained 60 for older adults because adverse-outcome risk rises below that level. It recommends checking how long the abnormality has lasted and whether albumin, a protein, is leaking into urine. When cystatin C, another blood marker of filtration, is available, it recommends using both creatinine and cystatin C for a more accurate estimate. Age belongs in the risk discussion; it does not erase the current diagnostic framework.

2. Thyroid (TSH)

TSH, thyroid-stimulating hormone, is the standard screening test for thyroid function. Higher TSH usually means an underactive thyroid.

A 2007 analysis of US national survey data by Martin Surks and Joseph Hollowell found that the range of TSH readings moves steadily higher with age. They argued that a single reference range for all ages could label too many older people as having subclinical hypothyroidism: raised TSH with a normal thyroid-hormone level. The analysis compared age groups at one point in time; it did not show that any particular TSH is safe or should go untreated. The age-specific figures are in the evidence details below.

In practice, TSH is interpreted with symptoms, free thyroxine, medications, prior results and repeat testing when appropriate. The US Preventive Services Task Force (USPSTF) finds insufficient evidence to determine the balance of benefits and harms of screening nonpregnant adults without symptoms; that is not a recommendation to ignore symptoms or an abnormal result.

3. Does blood sugar (HbA1c) rise with age?

HbA1c reflects glucose exposure over roughly two to three months. The American Diabetes Association’s (ADA) 2026 Standards of Care retain 6.5% as the diabetes threshold. Unless blood sugar is unmistakably high in the appropriate clinical setting, diagnosis requires confirmation.

In a 2008 Diabetes Care study of people without diabetes in two US population studies, HbA1c was modestly higher in older groups. The pattern remained after excluding people whose blood sugar rose abnormally during a glucose-tolerance test. The study describes differences between age groups, not how much one person’s HbA1c will rise each year.

The authors called for research on age-specific criteria. Current diabetes thresholds have not changed by age. The ADA also cautions that anemia, changes in how quickly red blood cells are replaced, kidney failure, dialysis, cirrhosis (advanced liver scarring) and other conditions can disrupt the relationship between HbA1c and glucose; in those settings, diagnosis may need to rely on blood-glucose tests instead.

4. Hemoglobin and anemia

Hemoglobin carries oxygen in red blood cells. Low hemoglobin is anemia.

In NHANES III data collected from 1988 to 1994, Jack Guralnik and colleagues found that anemia became more common after age 50 and exceeded 20% among people 85 and older. These figures came from a survey conducted decades ago, not a current forecast for every older adult.

That analysis did not establish aging itself as the cause of anemia. In it, roughly one-third of cases involved nutrient deficiency, one-third involved chronic inflammation or kidney disease, and one-third remained unexplained within the available survey data. Vitamin B12 is one possible contributor; the National Institute on Aging advises that some older adults may need fortified foods or a supplement after discussing their needs with a clinician.

How would you distinguish a finding that becomes more common with age from one explained by age alone?

Should lab ranges change with age?

This is where a seemingly simple question (should the range move with age?) turns into a genuine tug-of-war among researchers.

Marker	How it drifts with age	Risk of using young-adult ranges	Risk of age-adjusting
eGFR	Tends to fall	May increase diagnoses in older adults	Conflicts with current KDIGO guidance and could miss risk
TSH	Tends to rise	Could label age-associated readings as thyroid dysfunction	Could miss real thyroid disease
HbA1c	Rises slightly	Could flag more older adults	Thresholds unchanged; outcome data lacking
Hemoglobin	Falls (anemia more common)	Flags anemia that often has a cause	Could normalize treatable deficiencies

The table shows why the debate is hard. Age-adjusted ranges might reduce some biochemical labeling, but current decision thresholds are also tied to risk. And anemia shows why “common with age” cannot be translated into “harmless.”

What might an age-adjusted range clarify, and what might it conceal?