You're sitting with your lab results open on your phone, and there's a row of numbers highlighted in bold with a small "H" or "L" next to them. Your appointment isn't for another four days. You want to know right now whether you should be worried.
Blood test reports are dense by design. They pack a lot of clinical information into a narrow format built for physicians, not patients. But once you understand the basic structure and the most common tests, the results become much easier to process.
The Reference Range and What H or L Flags Mean
Every lab result appears next to a reference range, a span of numbers that represent typical values for a healthy adult. If your result falls outside that range, it gets flagged with an "H" for high or "L" for low.
It's worth understanding that reference ranges are statistical averages. They're calculated from large samples of presumably healthy people. About 5% of healthy people will have at least one value outside the normal range simply by chance. A flagged result is a signal worth discussing with your doctor, not an automatic cause for alarm.
The significance of a flagged result depends heavily on how far outside the range it falls, your age, your other results, your symptoms, and your medical history. A result that looks alarming in isolation might be entirely expected given the full picture.
The Complete Blood Count (CBC) Explained
The complete blood count, or CBC, is one of the most commonly ordered blood tests. It measures different types of cells in your blood and gives a broad picture of your overall health.
WBC stands for white blood cell count. White blood cells are your immune system's soldiers. A high WBC can signal infection, inflammation, or in some cases, an immune disorder. A low WBC might indicate a bone marrow issue or the effect of certain medications.
RBC stands for red blood cell count. Red blood cells carry oxygen through your body. A low RBC, combined with low hemoglobin, often indicates anemia, which is a condition where your blood isn't carrying enough oxygen.
Hemoglobin is the protein inside red blood cells that actually binds to oxygen. Hematocrit is the percentage of your blood volume made up of red blood cells. Both of these values move together and help your doctor assess how well your blood is oxygenating your body.
Platelets are the tiny cells that help your blood clot when you have a cut or injury. Low platelets can mean a higher risk of bleeding. High platelets can point to inflammation or, less commonly, a blood disorder.
The Basic Metabolic Panel
The basic metabolic panel, or BMP, is a group of eight tests that give your doctor a snapshot of your kidneys, blood sugar, and electrolyte balance.
Glucose measures the sugar in your blood at the time of the draw. Fasting glucose above 126 mg/dL on two separate tests is one of the criteria used to diagnose diabetes. Between 100 and 125 mg/dL is considered prediabetes range, which is reversible with lifestyle changes.
Creatinine and BUN (blood urea nitrogen) are both waste products your kidneys filter out of your blood. Elevated levels of either can indicate that your kidneys aren't clearing waste as efficiently as expected. Single elevated readings often warrant a repeat test before any conclusions are drawn.
Electrolytes, including sodium, potassium, chloride, and bicarbonate, help your body regulate fluid balance and nerve function. These values are rarely alarming on their own unless they're significantly outside range, which your doctor would typically address quickly.
Liver Function Tests
Liver function tests (LFTs) are a group of values that tell your doctor how well your liver is working.
ALT, which stands for alanine aminotransferase, is an enzyme found primarily in liver cells. When liver cells are damaged, ALT leaks into the blood and levels rise. Mildly elevated ALT can result from many things, including fatty liver, alcohol use, certain medications, or even intense exercise.
AST, or aspartate aminotransferase, is similar to ALT but is found in more organs, including the heart and muscles. An elevated AST alongside elevated ALT points more specifically toward the liver. AST alone being elevated can sometimes reflect muscle-related issues.
Bilirubin is a yellow substance produced when red blood cells break down. The liver processes it and removes it from the body. Elevated bilirubin can cause jaundice (yellowing of the skin or eyes) and can indicate liver or bile duct problems.
What "Borderline" or "Slightly Elevated" Actually Means
These phrases appear in doctor notes and lab comments, and they can be genuinely hard to interpret. Borderline typically means a result is close to the edge of the reference range, either just inside or just outside it.
A single slightly elevated result often prompts a repeat test rather than immediate treatment. Many values fluctuate day to day depending on hydration, food intake, time of day, recent activity, and other factors. Your doctor will look at the trend over time, not just the single number.
When to Call Your Doctor Before Your Appointment
Most flagged values are worth discussing at your scheduled appointment. But call sooner if your results include a critical value, which many labs mark clearly, if you're experiencing symptoms that are worsening, or if you received results for a test ordered urgently.
Your doctor's office can often give you a brief phone explanation to help you decide whether to come in sooner.
If you're looking at a lab report right now and want help understanding the specific values, ReportPlain lets you paste your results and get a plain-English explanation of what each item means and whether it's flagged. Nothing is stored, and it takes about a minute. It's a useful starting point before you talk to your doctor so the conversation can be more focused and productive.