What Do "High" and "Low" Lab Results Really Mean?

Disclaimer: This blog post is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional for any health concerns or before making any decisions related to your health or treatment.

That Sinking Feeling: Understanding "H" and "L" on Your Lab Report

You open your lab results, and your eyes immediately scan the page. Most of the numbers are in the clear, but then you spot it: a single result with a bold "H" (High) or "L" (Low) next to it.

It’s a sinking feeling, isn't it?

Your heart starts to beat a little faster. Your mind races. You're already opening a new browser tab to ask "Dr. Google" what a "high [test name]" means, and the results are... well, they're terrifying.

This is a completely normal reaction, and it’s one I see all the time.

As a doctor, here is the most important thing I want you to know: A "High" or "Low" flag on a lab test is almost never a reason for immediate panic.

These flags are not a diagnosis. They are not a final verdict. They are simply a signal a "check engine" light telling your doctor to take a closer look. It's common to feel this anxiety, so let's break down what these flags really mean.

What You'll Learn in This Post:

• What "H" and "L" flags actually are (and who puts them there)
• Common, everyday reasons your results might be out of range
• How your doctor investigates a result to see the full story
• What you should (and shouldn't) do after seeing an "H" or "L"

What Are "High" and "Low" Flags, Really?

In a previous post, we discussed "reference ranges." As a quick reminder, this range is where the results of 95% of a large, healthy population fall.

An "H" or "L" flag is usually an automated signal from the laboratory's computer. It simply means your number falls outside of that 95% average.

Think of it this way: If your car's "check engine" light comes on, it doesn't mean the engine is about to explode. It could be something serious, but it could also just be a loose gas cap. The light's only job is to tell you, "Hey, a mechanic should probably look at this."

Lab flags work the same way. They are just the computer's way of telling your doctor, "Hey, this one is statistically outside the average. You might want to look at it."

5 Common Reasons for an "Out of Range" Result

Before you jump to the worst-case scenario, it's important to know that dozens of harmless, everyday factors can push a result slightly out of the "normal" range.

Here are a few of the most common ones:

1. What You Ate or Drank
This is a big one. If you had a test for glucose (blood sugar) but you drank a sugary coffee an hour earlier, your result will almost certainly be flagged as "H". This doesn't mean you have diabetes; it means the test wasn't done under the right conditions (which is usually fasting).

2. The Time of Day
Your body runs on a 24-hour clock, and so do your hormones. A great example is cortisol (your body's main stress hormone). It is supposed to be "H" in the morning to wake you up and "L" at night so you can sleep. A "high" cortisol result at 8 AM is perfectly normal.

3. A Minor, Temporary Sickness
If you have a White Blood Cell (WBC) count that's flagged as "H," it could simply mean your body is doing its job! Your WBCs are your immune system's army. That "H" often just shows that your body is busy fighting off a simple cold or minor infection.

4. Medications and Supplements
Many common medications can affect your lab work. For example, some blood pressure medicines (diuretics, or "water pills") can sometimes make your potassium level "L". Even over-the-counter supplements like Biotin (a B-vitamin) can interfere with certain hormone tests, giving false results.

5. Your Own Personal "Normal"
Remember, the reference range only covers 95% of people. You might be one of the perfectly healthy 5% whose "normal" baseline is just naturally a little high or low for that test.

 

How Your Doctor Investigates (What We're Really Looking For)

When I see an "H" or "L" on your report, I don't just look at that single number. My job is to be a medical detective and put that clue into the context of your whole story.

Here’s what I’m looking for:

The Big Picture: You. How do you feel? An "L" thyroid result in a patient who feels energetic and well is very different from an "L" thyroid result in a patient who is exhausted, cold, and gaining weight. Your symptoms are the most important clue.

The Trend: We Look at Your History. This is the most powerful tool we have. Is this "H" result brand new? Or have you been on the high side of normal for the past 10 years? A sudden change is far more interesting to us than a number that's always been your baseline.

The Pattern: We Look at Related Tests. We almost never make a decision based on one test. We look for patterns. For example, if one of your liver enzymes is "H" but all your other liver tests are perfect, it's often not a concern. If several liver tests are "H" all at once, that's a pattern that tells us to investigate further.

When to See a Doctor (And What Not to Do)

This is the most important part. Seeing an "H" or "L" is not a medical emergency, but it is not something to ignore. Your non-negotiable next step is to discuss the result with a healthcare professional.

Here is the right way to handle it:

• DO NOT Panic. A single out-of-range number is just a piece of data, not a diagnosis.
• DO NOT "Google" Yourself into an Anxiety Spiral. Internet searches will always give you the scariest, worst-case possibilities.
• DO NOT Try to "Fix" the Number Yourself. Please, do not start or stop any medications, diets, or supplements to try and change your lab result. This can be confusing and sometimes even dangerous.
• DO Call Your Doctor's Office. Make a non-emergency appointment (or send a message through your patient portal) to review the results. This is the calm, correct, and effective next step.

The only exception: If you get a lab result and you are having severe, new symptoms (like chest pain, difficulty breathing, sudden weakness, or intense pain), you should seek emergency care.(alert-warning)

Your Lab Report is a Conversation Starter

Your lab results are not a report card. You don't "pass" or "fail."They are an incredible tool that gives us a glimpse inside your body. An "H" or "L" is simply the start of a conversation, not the end of one. So, take a deep breath. Close the "Dr. Google" tab. And bring your questions to the person who is trained to interpret them with you: your doctor. Together, we can turn that piece of data into a meaningful decision for your health.