Your lab results compare each measured value against a reference range — the normal span for that test. A value inside the range reads as normal; one outside it is flagged H (high) or L (low). An H or L flag does not automatically mean something is wrong — many are mild, expected variations. What matters is which value is off, by how much, and your overall health picture.
You open the patient portal and there it is — a wall of acronyms, numbers and little flags marked H or L. No explanation, no context and usually no doctor in the room to ask. It is easy to assume the worst when you cannot tell a harmless variation from something that actually matters.
This guide breaks down the most common lab tests in plain English so you walk into your next appointment informed instead of anxious.
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What does H and L mean on lab results?
H means High and L means Low. The lab compares each of your values to a reference range — the band of results seen in most healthy people — and flags anything that falls above or below it.
A flag is not a diagnosis. Results sit outside the range for all kinds of harmless reasons — a recent meal, hard exercise, dehydration, a medication or even the time of day. Reference ranges also vary slightly from lab to lab, so a value flagged at one lab might be normal at another.
What matters is context — how far outside the range you are, whether the value is trending over time and how it fits with your symptoms and history. A number barely past the line is rarely the same as one far beyond it. Your provider reads the whole picture not a single letter.
What do the most common lab tests mean?
CBC — Complete Blood Count
Looks at the cells that make up your blood.
WBC (white blood cells)
Your immune system’s defenders. High can mean infection or inflammation. Low can mean your body is run down.
RBC (red blood cells)
Carry oxygen around your body. Low can point to anemia. High can mean dehydration or other causes.
Hemoglobin
The protein in red cells that actually carries oxygen. Low hemoglobin is the classic sign of anemia and can leave you tired or short of breath.
Hematocrit
The percentage of your blood made up of red cells. It moves together with hemoglobin and tells a similar story.
Platelets
Tiny cells that help your blood clot. Low can mean easier bruising or bleeding. High can raise clotting risk.
BMP / CMP — Metabolic Panel
Checks your blood sugar, kidneys, electrolytes and liver.
Glucose
Your blood sugar. High can point toward diabetes or a recent meal. Low can make you shaky or lightheaded.
Creatinine
A waste product your kidneys filter out. Higher levels can mean your kidneys are not clearing it as well as they should.
BUN (blood urea nitrogen)
Another kidney and hydration marker. It rises with dehydration or reduced kidney function.
Sodium
A key electrolyte that balances your fluids. Too high or too low can affect how you feel and think.
Potassium
An electrolyte your heart and muscles depend on. Levels that drift too far in either direction can affect your heartbeat.
ALT / AST
Liver enzymes. When they are elevated it can signal that the liver is irritated or stressed.
Lipid Panel — Cholesterol
Measures the fats in your blood that affect heart health.
Total cholesterol
The overall amount of cholesterol in your blood. It is a starting point not the whole picture.
LDL (“bad” cholesterol)
The type that can build up in your arteries. Lower is generally better.
HDL (“good” cholesterol)
The type that helps clear cholesterol away. Higher is generally better.
Triglycerides
A fat in your blood tied to diet, weight and activity. High levels can raise heart risk.
Thyroid Panel
Shows how well your thyroid is regulating your metabolism.
TSH
The signal your brain sends to your thyroid. High TSH often means an underactive thyroid. Low TSH often means an overactive one.
T3
One of the active thyroid hormones that sets your metabolism. It is read alongside TSH for the full picture.
T4
The main hormone your thyroid produces. Together with TSH and T3 it shows whether your thyroid is balanced.
Red flags — when to call your doctor
Most flagged values are not emergencies. A few are worth a prompt call to your provider — especially if you also feel unwell. Trust your symptoms and reach out about:
A glucose that is very high or very low especially with symptoms like confusion, fainting or extreme thirst
Potassium far outside the normal range since it can affect your heart rhythm
Hemoglobin that has dropped sharply or is very low along with dizziness or shortness of breath
Platelets that are very low together with unexplained bruising or bleeding
Creatinine or BUN that has climbed quickly which can mean your kidneys need attention
Any value your lab marks as “critical” or “panic”
If you have severe symptoms such as chest pain, trouble breathing, fainting or confusion do not wait for a callback — call 911 or go to the nearest emergency room.
Questions to ask your doctor about your lab results
1Which of my results are outside the normal range and by how much?
2Is anything here something I need to act on now or just watch over time?
3Could a medication, my diet or how I prepared for the test have affected these numbers?
4Do any of these results change my diagnosis or treatment plan?
5When should I repeat this test to see if anything has changed?
6Are there warning signs I should watch for before my next visit?
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Medical disclaimer: This page is educational and is not medical advice, diagnosis or treatment. Lab results must be interpreted by a licensed healthcare provider who knows your full history. Always follow your provider’s guidance and call 911 in an emergency.