Medical disclaimer: This article is for educational purposes only and is not a substitute for professional medical advice. Always discuss your blood test results with your doctor, who can interpret them in the context of your complete medical history, symptoms, and risk factors.
You get blood work done. A few days later, a PDF arrives in your patient portal with dozens of numbers, abbreviations, and reference ranges. Some are flagged high, some flagged low, and the results page offers zero explanation of what any of it means. You Google a few values, read three terrifying articles, and wait nervously for your doctor's appointment.
This guide is designed to replace that experience. It explains what each major blood test measures, what the normal ranges mean, and when an out-of-range result is a genuine concern versus a normal variation. The goal is not to replace your doctor — it is to help you walk into that appointment informed and ready to ask the right questions.
Most routine blood work involves some combination of these standard panels:
| Range (mg/dL) | Classification | What It Means |
|---|---|---|
| 70–99 | Normal | Healthy blood sugar regulation |
| 100–125 | Prediabetes | Impaired fasting glucose; early insulin resistance |
| 126+ | Diabetes | Requires confirmation with a second test |
| Below 70 | Hypoglycemia | Abnormally low; may cause symptoms |
Fasting glucose is a snapshot of your blood sugar at one moment in time. It can be affected by what you ate the night before, stress, sleep quality, and even the time of day. A single elevated reading does not mean you have diabetes — but it warrants follow-up testing, typically an A1C test for a longer-term picture. Use the Blood Sugar Converter to convert between mg/dL and mmol/L if your lab uses different units.
| A1C Level | Classification | Estimated Average Glucose |
|---|---|---|
| Below 5.7% | Normal | ~117 mg/dL or lower |
| 5.7–6.4% | Prediabetes | ~117–137 mg/dL |
| 6.5%+ | Diabetes | ~140 mg/dL or higher |
A1C is one of the most important metabolic markers because it reflects average blood sugar over the lifespan of a red blood cell (approximately 2–3 months). A fasting glucose test can be normal while A1C reveals prediabetes, because A1C captures the post-meal blood sugar spikes that fasting glucose misses. Research published in the New England Journal of Medicine has shown that for every 1% reduction in A1C, there is approximately a 21% reduction in diabetes-related deaths and a 37% reduction in microvascular complications.
Optimal vs normal: Many longevity-focused physicians distinguish between "normal" (within the lab reference range) and "optimal" (associated with the lowest disease risk). For A1C, the lab range says below 5.7% is normal, but some research suggests that A1C levels of 5.0–5.4% are associated with the lowest cardiovascular risk. A value of 5.6% is technically normal but is already trending toward metabolic dysfunction.
| Marker | Optimal | Borderline | High Risk |
|---|---|---|---|
| Total Cholesterol | Under 200 mg/dL | 200–239 | 240+ |
| LDL ("Bad") | Under 100 mg/dL | 100–159 | 160+ |
| HDL ("Good") | 60+ mg/dL | 40–59 | Under 40 |
| Triglycerides | Under 150 mg/dL | 150–199 | 200+ |
Source: American Heart Association and National Heart, Lung, and Blood Institute guidelines.
LDL cholesterol is the primary driver of atherosclerosis (plaque buildup in arteries). The lower, the better, especially for people with existing cardiovascular risk factors. For those with heart disease or very high risk, guidelines recommend LDL under 70 mg/dL, sometimes achieved with statin medications.
HDL cholesterol is protective — it helps remove LDL from arteries. Exercise, moderate alcohol consumption, and certain dietary fats (olive oil, avocado, nuts) tend to raise HDL. Values above 60 mg/dL are considered cardioprotective.
Triglycerides are strongly influenced by diet, particularly refined carbohydrates, sugar, and alcohol. Elevated triglycerides (above 150 mg/dL) are an independent risk factor for heart disease and are also a marker of insulin resistance and metabolic syndrome.
The ratios matter: Many cardiologists consider the total cholesterol to HDL ratio more predictive than any individual number. A ratio below 3.5:1 is considered excellent. For example, total cholesterol of 210 with HDL of 70 gives a ratio of 3.0 — which is actually quite good despite the total being above 200. Triglyceride-to-HDL ratio is another useful metric: values under 2.0 suggest good insulin sensitivity, while values above 3.0 suggest insulin resistance even if individual numbers are in range.
| Marker | Normal Range | What Elevation May Indicate |
|---|---|---|
| ALT (alanine aminotransferase) | 7–56 U/L | Liver inflammation, fatty liver, medication effects |
| AST (aspartate aminotransferase) | 10–40 U/L | Liver or muscle damage (less liver-specific than ALT) |
| ALP (alkaline phosphatase) | 44–147 U/L | Bile duct issues, bone disorders |
| Bilirubin | 0.1–1.2 mg/dL | Liver dysfunction, gallstones (high levels cause jaundice) |
| Albumin | 3.5–5.5 g/dL | Low values may indicate liver disease or malnutrition |
ALT is the most liver-specific of these enzymes. Mildly elevated ALT (56–100 U/L) is extremely common and is frequently caused by nonalcoholic fatty liver disease (NAFLD), which affects an estimated 25–30% of American adults according to research published in the journal Hepatology. Other common causes include certain medications (statins, acetaminophen, antibiotics), alcohol, and intense exercise.
A single mildly elevated reading is not cause for alarm but should be monitored. Persistently elevated liver enzymes or values more than 2–3 times the upper limit of normal warrant further investigation, potentially including imaging (ultrasound) and additional lab work.
| Marker | Normal Range | What It Measures |
|---|---|---|
| Creatinine | 0.7–1.3 mg/dL (men), 0.6–1.1 (women) | Waste product filtered by kidneys; reflects kidney function |
| BUN (blood urea nitrogen) | 6–20 mg/dL | Waste product from protein metabolism; reflects kidney and liver function |
| eGFR (estimated glomerular filtration rate) | 90+ mL/min | How well kidneys filter; calculated from creatinine, age, sex |
eGFR is the most clinically useful kidney marker. It estimates how many milliliters of blood your kidneys filter per minute. Values above 90 are normal. Values between 60–89 may indicate mild kidney function decline (common with aging and not necessarily a problem). Below 60 suggests moderate kidney impairment and warrants monitoring and lifestyle changes. Below 15 indicates kidney failure.
Important context: creatinine levels are affected by muscle mass. A very muscular person may have a creatinine of 1.4 mg/dL that is completely normal for their body composition, while the same value in a small-framed person could indicate reduced kidney function. Creatine supplementation can also raise creatinine levels without indicating any kidney problem. Always interpret creatinine in conjunction with eGFR.
| Marker | Normal Range | What It Measures |
|---|---|---|
| WBC (white blood cells) | 4,500–11,000 /µL | Immune system activity; high may indicate infection |
| RBC (red blood cells) | 4.5–5.5 million /µL (men), 4.0–5.0 (women) | Oxygen-carrying capacity |
| Hemoglobin | 13.5–17.5 g/dL (men), 12.0–16.0 (women) | Protein in RBCs that carries oxygen; low = anemia |
| Hematocrit | 38.3–48.6% (men), 35.5–44.9% (women) | Percentage of blood that is RBCs |
| Platelets | 150,000–400,000 /µL | Blood clotting cells |
The CBC is the broadest screening tool in blood work. Hemoglobin is the most commonly flagged value — low hemoglobin (anemia) causes fatigue, weakness, and shortness of breath and is frequently caused by iron deficiency, especially in women of reproductive age. Anemia can also result from vitamin B12 or folate deficiency, chronic disease, or bone marrow disorders.
| Marker | Normal Range | What Abnormal Values Suggest |
|---|---|---|
| TSH | 0.4–4.0 mIU/L | High TSH = underactive thyroid (hypothyroid); Low TSH = overactive (hyperthyroid) |
| Free T4 | 0.8–1.8 ng/dL | Low = hypothyroid; High = hyperthyroid |
| Free T3 | 2.3–4.2 pg/mL | Active thyroid hormone; helps clarify T4 results |
TSH (thyroid-stimulating hormone) is the standard screening test. The relationship is inverse and counterintuitive: high TSH means your thyroid is underactive (the pituitary gland is producing more TSH to try to stimulate a sluggish thyroid). Hypothyroidism is very common, affecting an estimated 5% of Americans, and causes symptoms including fatigue, weight gain, cold intolerance, and dry skin. It is easily treated with thyroid hormone replacement medication.
The "normal" range debate: The TSH reference range of 0.4–4.0 is controversial. Many endocrinologists consider TSH above 2.5 to be suboptimal, and some patients with TSH of 3.0–4.0 experience hypothyroid symptoms that improve with treatment. If your TSH is in the upper end of "normal" and you have symptoms, it is worth discussing with your doctor.
hs-CRP (high-sensitivity C-reactive protein) measures systemic inflammation. Levels below 1.0 mg/L indicate low cardiovascular risk, 1.0–3.0 mg/L indicates moderate risk, and above 3.0 mg/L indicates higher risk. CRP is a nonspecific marker — it rises with any inflammation, including infections, injuries, and autoimmune conditions. A single elevated reading should be retested after any acute illness has resolved.
ESR (erythrocyte sedimentation rate) is another general inflammation marker, often ordered alongside CRP. Normal is typically 0–20 mm/hr for men and 0–30 mm/hr for women, though it naturally increases with age.
Individual markers tell part of the story. The full picture emerges from patterns across multiple markers. Metabolic syndrome — a cluster of conditions that dramatically increases cardiovascular and diabetes risk — is diagnosed when three or more of the following are present:
According to the National Health and Nutrition Examination Survey (NHANES), approximately 35% of American adults meet the criteria for metabolic syndrome. The condition is largely driven by insulin resistance and is highly responsive to lifestyle changes — particularly exercise, reduced refined carbohydrate intake, and weight loss. Track your overall health trajectory with the Biological Age Calculator.
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