How to read a blood test result: reference ranges, units and deviations

What makes up a blood test result

Most Lithuanian laboratories present results as a table. Each row usually contains five pieces of information:

• Name of the marker — for example, hemoglobin, glucose or TSH. Labs sometimes also show a short code or abbreviation (Hb, GLU).
• Result — a number or a verbal reading ("negative", "positive", "inconclusive").
• Unit of measurement — mmol/L, g/L, µmol/L, U/L and others. The same marker can be measured in different units in different countries or labs.
• Reference range — two boundaries between which a result is considered typical for that lab and that group of patients.
• Flag — an arrow, an asterisk, or the letters H (high) and L (low), shown when a result falls outside the reference range.

It is also worth checking the small print: the test date, the time the sample was taken, and any notes about sample quality (for example, hemolysis) — these can explain an unexpected result.

What a reference range is and why it differs between labs

A reference range is the span of values within which most healthy people's results fall. According to MedlinePlus, the U.S. National Library of Medicine's patient-information service, reference ranges are built from test results of large groups of healthy people, so they reflect an average rather than a strict line between health and disease. The same test can have several ranges — separate ones for children and adults, for men and women, or for pregnant women.

More importantly, laboratories use different testing methods and reagents, so their reference ranges differ too. Because of this, MedlinePlus highlights two practical points:

• Compare your result only to the reference range printed on that same result sheet, not to norms found elsewhere.
• Results from different laboratories should not be compared directly with each other — to track a change over time, it is best to test at the same laboratory.

This is why Proactiva's marker pages also list reference norms with a source and date, while reminding readers on every page that the definitive range is the one printed on their own result sheet.

What a result outside the norm means

A result outside the reference range is not a diagnosis. MedlinePlus notes that healthy people's results sometimes fall outside the reference range — this is normal, and a doctor evaluating such a result also looks at other information: symptoms, examination findings, medical and family history, and earlier test results. Equally, a result within range is not a guarantee that everything is fine: some conditions can be present even with "normal" numbers, so further testing may be needed if symptoms are present.

In practice, it helps to distinguish three situations:

• A small deviation with no symptoms — often the test is simply repeated, or the marker is monitored over time. What a raised or lowered value of a specific marker means is explained on that marker's own page, for example cholesterol or ferritin.
• A deviation together with symptoms — for example, prolonged fatigue combined with low hemoglobin. In this case a doctor decides on the panel of tests and next steps; which tests are typically ordered for a given symptom is covered in Proactiva's "Which tests to get" section (for example, for fatigue).
• Several related markers deviating together — for a doctor this is often more informative than a single number, because markers are assessed in groups (for example, liver enzymes or a lipid panel).

In every case, interpreting results is a doctor's job — making decisions on your own based on the result sheet is not advisable.

Units: mmol/L, mg/dL, and why the numbers look different

MedlinePlus's second point concerns units. The same marker can be measured in different units, much like a recipe can be written in teaspoons or in grams. Lithuanian laboratories mostly use the SI system (mmol/L, µmol/L, g/L), while foreign sources and some apps use mg/dL, ng/mL or pg/mL. For example, a glucose level of 5.5 mmol/L and 99 mg/dL are the same value, and total cholesterol of 5.2 mmol/L corresponds to about 200 mg/dL.

Before comparing a result to any norm, make sure the units match. If they don't, you can convert them using Proactiva's unit converter, which lists the conversion factors and their sources.

What can skew a result

Accuracy depends not only on the laboratory but also on preparation. According to MedlinePlus, most blood tests need no special preparation, but some — glucose, a lipid panel (cholesterol and triglycerides), a basic metabolic panel — require fasting: typically not eating or drinking anything except plain water for 8–12 hours beforehand. During the fasting period you should avoid chewing gum, smoking and exercising; the exact fasting time should be confirmed with the doctor who ordered the test. Proactiva's "How long to fast" calculator can help you check how many hours to fast before a specific test.

A result can also be affected by medications and supplements you take, intense physical exertion before the test, alcohol, pregnancy, or an acute infection. It is worth telling your doctor about all medications and supplements you take before a test — they should not be stopped on your own initiative.

False positive and false negative results

No test is perfect. MedlinePlus distinguishes a false positive result (the test shows a condition that isn't there) from a false negative (the test misses a condition that is there). Such errors are rare, but more common with certain tests or when a sample is collected or stored incorrectly. If a doctor suspects a result may be inaccurate, the test is repeated. So one unexpected number is not a reason to panic — it's a reason to repeat the test and discuss it.

How to track results over time

A single test is a snapshot. Far more informative is the trend: whether a marker is stable, rising or falling over several years. For a reliable trend, MedlinePlus advises testing at the same laboratory and comparing only results from that same lab. In practice it helps to:

• keep your results in one place (paper or digital) with dates;
• note the conditions under which you were tested (fasting or not, whether you were ill at the time);
• repeat the same markers if your doctor recommends monitoring them.

Free tests in Lithuania

In Lithuania, some preventive tests are provided free of charge, through a family doctor, to people covered by mandatory health insurance, under prevention programmes funded by the Valstybinė ligonių kasa (the National Health Insurance Fund, Lithuania's compulsory health insurance administrator). For example, people aged 40–60 inclusive can, under the cardiovascular disease prevention programme, periodically have their blood glucose, cholesterol and triglyceride levels checked free of charge, along with an electrocardiogram. The conditions of all such programmes, with sources, are collected in Proactiva's Prevention section. Outside these programmes, a family doctor orders tests based on clinical indications, and without a referral, tests can be done for a fee at private laboratories — indicative prices are listed on the individual marker pages.

When to see a doctor without waiting

It is always worth discussing a test result with your doctor, but in some cases it is not advisable to wait for a routine appointment: when the laboratory flags a result as critical, when a large deviation is accompanied by symptoms (severe weakness, shortness of breath, chest pain, fever), or when a result has changed unexpectedly and sharply from a previous one. In such cases contact your family doctor, and for acute symptoms, call 112 (the emergency number in Lithuania).