Biomarkers

How to Read Blood Test Results: A Step-by-Step Guide

Read any blood test report in five steps: the test name, your result, the units, the reference range, and the flag. Includes an annotated example report and an abbreviations glossary.

Published

March 17, 2026

Last updated

October 5, 2026

Read time

13 min read

Medically reviewed by

Rhonda Collins, FNP-C

By Kenneth Lou , Co-Founder and CEO

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To read blood test results, take each line of the report in the same order:

1. Test name. Find what was measured, often as an abbreviation such as Hgb, ALT, or TSH. The glossary below translates the common ones.
2. Result. Read your value for that test.
3. Units. Check the units beside the result, such as mg/dL, mmol/L, or %. A number only means something in its units.
4. Reference range. Compare your result with the range printed on the same report. That range comes from the laboratory that ran your sample and applies to that report.
5. Flag. Look for H (high), L (low), or a critical marker. A flag says the result is outside the lab’s range. It does not say why, and it is not a diagnosis.

Then read flagged results together with the related tests on the report and with your earlier results. The sections below explain each part of a report, walk through an annotated example, and cover what to do when a result is out of range.

On this page

• Anatomy of a blood test report
• An annotated example report
• What H, L, and critical flags mean
• Why reference ranges differ between labs
• Blood test abbreviations glossary
• Common panels at a glance
• How to read results over time
• Reading results in a patient portal or MyChart
• What to do with an out-of-range result
• Frequently asked questions

Anatomy of a blood test report

Most blood test reports share the same layout, whether they come from a hospital, a national lab, or a patient portal. Federal lab rules (42 CFR 493.1291) require a report to show the test performed, the result, and the units of measurement where they apply. The lab must also make its reference intervals available. In practice, a report usually contains:

• Header details: your name and identifiers, the ordering clinician, and the collection and report dates. The collection date is the one to use when you compare results over time.
• Performing lab: the name and address of the laboratory that ran the test. This tells you whose reference ranges apply.
• Panel headings: tests are grouped by panel, such as a complete blood count (CBC) or a comprehensive metabolic panel (CMP).
• Test name column: the marker measured, often abbreviated.
• Result column: your value. Some reports show a flagged result in a separate column or in bold.
• Flag column: H, L, or a critical marker when the result falls outside the range.
• Units column: the measurement units for that result.
• Reference range column: the lab’s range for that test, sometimes adjusted for sex or age.
• Comments: notes about the sample, such as hemolysis (damaged red cells in the sample) or a result that was repeated, and footnotes explaining how a range was set.

Some results are not numbers. Qualitative tests report words such as positive, negative, reactive, or not detected. MedlinePlus explains that a positive or abnormal result means the test found what it was looking for, and that any test can produce false positive and false negative results.

An annotated example report

The report below is fictional. The values and ranges are invented for illustration and are not reference values. Use the ranges printed on your own report.

How to read each flagged line in this fictional report:

• White blood cells, 11.4 H. The count is above this lab’s range. A single high white cell count is read alongside the white cell differential, any recent illness, and earlier counts. The CBC results guide covers how to read each CBC line and its flags in detail.
• Glucose, fasting, 104 H. The result is above the lab’s range. The CDC also lists a fasting glucose of 100 to 125 mg/dL as being in the prediabetes range, and notes that a doctor may retest to make sure results are accurate. This line only applies if you actually fasted, which is why preparation matters (see fasting for blood work).
• HbA1c, 5.8% H. The range here is a clinical cutoff rather than a spread of healthy values. The CDC defines an A1C of 5.7% to 6.4% as the prediabetes range. HbA1c and fasting glucose measure related things in different ways, so reading them together is more informative than either alone.
• Vitamin D, 24 L. The result is below this lab’s range. Labs do not all use the same vitamin D range, so compare it with the range on your own report rather than one found elsewhere.

The unflagged lines still matter. Hemoglobin, potassium, ALT, and TSH sit inside their ranges here, and they become more useful once you can compare them with your next result.

What H, L, and critical flags mean

An H flag means the result is above the lab’s reference range. An L flag means it is below. Some reports use A or an asterisk for abnormal, or print the result in a separate “out of range” column. A flag is a sorting aid: it tells you where to look, not what is wrong.

A flag does not always mean a problem. Reference ranges are usually set to cover the middle 95% of results from a healthy reference group. A review in the Journal of Clinical Pathology notes that this places 5% of healthy values outside the range by design. Because a few common panels together report around 20 analytes, it is statistically likely that at least one result on a routine report will be flagged in a healthy person.

A critical value (also called a panic or alert value) is different. It is a result the lab treats as potentially life threatening. Under federal lab rules, the laboratory must immediately alert whoever ordered the test when a result indicates an imminently life-threatening condition or reaches a panic or alert value. Each lab sets its own list of critical tests and thresholds, so there is no single universal cutoff to compare against. If your report or portal labels a result as critical, or the lab or your clinician contacts you about it, follow their instructions promptly.

Why reference ranges differ between labs

Reference ranges differ because labs use different equipment, testing methods, and reference populations. MedlinePlus notes that because labs often use different methods, they often use different reference ranges, and that you cannot directly compare results from different labs. Three practical consequences follow:

• Use the range on your own report. A range copied from a website or another lab may not apply to your sample.
• Watch the units. The same marker can be reported in different units. Glucose, for example, appears in mg/dL on most US reports and in mmol/L in many other countries, so the numbers look very different for the same blood sugar.
• Know which kind of range you are reading. Most ranges describe where healthy people usually fall. Some, such as HbA1c and fasting glucose, are clinical cutoffs taken from guidelines. Ranges can also be adjusted for sex, age, or pregnancy, so two people on the same report can have different ranges for the same test.

Some websites and services also publish narrower “optimal” ranges. These are not standardized, so the same marker can have different optimal ranges depending on the source. Ask what evidence a given optimal range is based on before acting on it.

Blood test abbreviations glossary

Each abbreviation links to Mito’s reference page for that marker, which explains what it measures and what high and low results can mean.

Complete blood count (CBC). For line-by-line CBC interpretation, see the CBC results guide.

• WBC: white blood cell count
• RBC: red blood cell count
• Hgb or Hb: hemoglobin
• Hct: hematocrit
• MCV: mean corpuscular volume
• MCH: mean corpuscular hemoglobin
• MCHC: mean corpuscular hemoglobin concentration
• RDW: red cell distribution width
• PLT: platelet count
• NEUT and LYMPH: neutrophils and lymphocytes

Metabolic panel (BMP or CMP).

• GLU: glucose
• BUN: blood urea nitrogen
• Cr or CREAT: creatinine
• eGFR: estimated glomerular filtration rate
• Na, K, Cl: sodium, potassium, and chloride
• CO2 or HCO3: carbon dioxide (bicarbonate)
• Ca: calcium
• ALB and TP: albumin and total protein
• T. Bili: total bilirubin
• ALT and AST: alanine transaminase and aspartate transaminase
• ALP: alkaline phosphatase
• GGT: gamma-glutamyl transferase

Lipids and heart markers.

• TC: total cholesterol
• LDL-C: LDL cholesterol
• HDL-C: HDL cholesterol
• TG: triglycerides
• Non-HDL-C: non-HDL cholesterol
• ApoB: apolipoprotein B
• Lp(a): lipoprotein(a)
• hs-CRP: high-sensitivity C-reactive protein

Blood sugar, thyroid, and nutrients.

• HbA1c or A1C: hemoglobin A1c
• TSH: thyroid stimulating hormone
• FT4 and FT3: free thyroxine and free triiodothyronine
• ESR: erythrocyte sedimentation rate
• Ferritin: ferritin
• 25-OH vitamin D: vitamin D
• B12: vitamin B12

Common panels at a glance

Most routine blood work combines a few standard panels. Each one answers a different question, so a flag is easiest to interpret within its own panel.

• Complete blood count (CBC): red cells, white cells, and platelets. Read the related lines in each group together. The CBC results guide explains every line.
• Comprehensive metabolic panel (CMP): glucose, kidney markers, electrolytes, liver enzymes, and proteins. The metabolic panel explainer walks through each group.
• Lipid panel: total, LDL, and HDL cholesterol and triglycerides. Some reports add calculated values such as non-HDL cholesterol or the TG:HDL-C ratio.
• Thyroid tests: usually TSH, sometimes with free T4 and free T3. The TSH results guide explains how TSH and free T4 are read together.
• HbA1c and fasting glucose: two views of blood sugar, read against the CDC’s clinical cutoffs described above.
• Inflammation and nutrient markers: such as hs-CRP, ferritin, vitamin D, and vitamin B12. Each has its own reference page in the glossary above.

How to read results over time

A single result is a snapshot. A series of results shows direction, which is often more useful than whether one value crossed a line.

• Compare like with like. Use results from the same lab where possible, because ranges and methods differ between labs. If the lab changed, compare each result with its own range rather than comparing the raw numbers.
• Match the conditions. MedlinePlus lists food and drink, medicines and supplements, hard exercise before the test, menstrual timing, and whether you followed preparation instructions as factors that can affect results. Similar timing and preparation make two results easier to compare.
• Look for a pattern, not a single step. A small change between two draws can reflect normal day-to-day variation. A steady drift across several results, or a change that lines up with new symptoms or a medication change, deserves more attention than one isolated flag.
• Keep your reports in one place. Gathering past results makes trends visible. Mito members can upload PDF reports from other providers to the dashboard so earlier results sit alongside new ones.

Reading results in a patient portal or MyChart

Many people now see results in a patient portal, such as MyChart or a lab’s own app, before anyone has explained them. Under the 21st Century Cures Act, many health systems release results to the portal as soon as they are final. In a 2023 survey of 8,139 patients at four US academic medical centers, about 96% preferred to receive results immediately through the portal, even before a clinician had reviewed them. Increased worry was more common among people whose results were abnormal (16.5%) than among those whose results were normal (5.0%).

A few habits make portal results easier to read:

• Open the full report, not only the summary list. The detailed view usually shows the units, the range, the flag, and the performing lab.
• Check whether the result is final. Portals may show preliminary or pending results that later change.
• Expect some results to arrive before others. Panels from the same draw can post at different times.
• Look for a clinician note. If none is attached, a flagged result may simply not have been reviewed yet.

What to do with an out-of-range result

Most out-of-range results are mild and isolated. Work through them in this order:

1. Confirm the basics. Check the units, the range on your own report, and whether the result is final.
2. Consider the conditions of the draw. Did you fast if the test needed it? Had you exercised hard, been ill, or taken a new medicine or supplement? These can all shift results.
3. Read the related results. A flag means more when related markers move with it, for example a low hemoglobin alongside low MCV, or a high fasting glucose alongside a high HbA1c.
4. Compare with earlier results. A value that has always sat slightly outside the range is different from one that has recently changed.
5. Decide on follow-up with a clinician. Retesting or further evaluation depends on the size of the change, the pattern, and your health history.

Contact a clinician promptly when a result is marked critical, when you are contacted about a result, when several related results are abnormal, when a result keeps moving in the same direction, or when an abnormal result comes with symptoms. Seek urgent care for severe symptoms rather than waiting for a routine appointment.

At Mito, a clinician on the care team reviews your results in the context of your full health picture before your analysis is finalized, and your action plan notes which markers to keep an eye on. The Mito Core Panel covers the CBC, metabolic panel, lipids, HbA1c, thyroid, and common nutrient markers in one draw.

Frequently asked questions

How often should I get blood work done?

There is no single schedule. How often to test depends on your age, health conditions, medications, and what your previous results showed. A result being tracked after a change is usually rechecked on a schedule set by whoever ordered the test.

Can I eat before getting blood work done?

It depends on the tests ordered. Some, such as fasting glucose and some lipid panels, are read differently when you have eaten, while many others are not affected. Follow the instructions for your specific order. The fasting for blood work guide covers which tests usually need it.

What should I do if a result is slightly outside the normal range?

A slightly out-of-range result is common and often not a sign of disease, because reference ranges are built to leave some healthy results outside them. Check the conditions of the draw, the related results, and your previous values, then use the steps in the out-of-range section above to decide on follow-up.

What is the difference between normal and optimal lab values?

A normal or reference range describes where most results from a healthy reference group fall, as set by the lab that ran your test. Optimal ranges are narrower targets published by some clinicians and services. They are not standardized, so check what evidence a given optimal range is based on.

Why do my results look different at a different lab?

Labs use different methods, equipment, and reference populations, so their ranges and sometimes their units differ. Compare each result with the range on its own report, and use the same lab for trends where you can.