“Normal” is not one number: Reference Ranges & Variability in Laboratory Testing

Have you ever been left scratching your head after stepping on the scale at the gym or the doctor’s office and learning your weight is different than it was yesterday when you weighed yourself on the bathroom scale? What causes this difference?

Well, potentially a lot of factors! Time of day (morning or evening) and scale mechanics (digital scale or sliding weights scale) are just two of the many factors that could be contributing to the difference you see. The same thing goes for laboratory testing results.

Repeated laboratory test results are seldom identical. But whoever said variety is the spice of life, clearly didn’t know how nerve-wracking it can be to see the slightest variation in your test results. We’re here to dive deeper into how your Everlywell at-home lab test results are reported as well as to discuss a few reasons for variance in laboratory testing results.

We’ll cover:

  • What can cause variance in laboratory testing results?
  • What is a reference range?
  • How are reference ranges determined?
  • How do health care providers use reference ranges?
  • How can you use the reference ranges in your report?

What can cause variance in laboratory testing results?

It’s not uncommon for results to vary when taking a repeat lab test, even if both tests were collected in a short time period, including the same day, performed at the same laboratory, and on the same instrument. This variability could be due to a number of factors.

For example, there is inherent lab variance due to the calibration of the lab instruments or the temperature your sample was stored (despite refrigeration being constant, there are many steps to get the sample ready for processing). Additionally, recent changes in your life−like your medication regimen, stress levels, and dietary habits−can cause your lab results to vary.

Test results can also vary based on the time of day you collect your sample. Hormones, like cortisol, are highest upon waking up in the morning. For premenopausal women certain hormone levels, like FSH (the hormone responsible for the growth and maturation of eggs) vary throughout a typical menstrual cycle. This is why the Everlywell Ovarian Reserve Test instructs you to collect your sample on Day 3 of your period when FSH levels are stable and provide a good baseline.

Baseline example 2

What is a reference range?

If you’ve used Everlywell’s at-home lab tests before, you may have noticed many of our tests provide a reference range in addition to your own level of a specific biomarker. Below in the image of the Everlywell results dashboard, you can see that the reference range for HDL, a type of cholesterol, is 40-100.

A reference range is the set of numbers that 95% of results from healthy individuals are expected to lie within. Each lab must define its own reference range for each type of test since collection kit materials, chemicals used to perform testing, lab instruments, and more can vary from lab to lab.

How are reference ranges determined?

CLIA-certified and CAP-accredited laboratories are required to establish their own reference range for each test or verify the reference range established by the company that makes the lab instrument used for the test.

Healthy and expected lab values differ from person to person based on a number of factors such as age, sex, height, weight, and genetics. To account for these differences in the reference range, laboratories need samples from a large enough group of people. This number is set by the particular lab. Everlywell Diagnostics for instance, uses a minimum of 120 samples when establishing reference ranges–this is well above the typical recommendation. For example, to establish a reference range for hemoglobin A1c (HbA1c), a biomarker that measures your average blood sugar over the past 2-3 months, a laboratory would need to collect and test blood samples from at least 120 people.

The reference range is the interval of values that 95% of the samples tested fell within. The reference range excludes the lower 2.5% of the population and the upper 2.5% of the samples tested. The bell-shaped, normal distribution looks like the graph below.

Graph example

Before we go on, we should note that the reference ranges for many hormone biomarkers, like estradiol and testosterone, are different for males than females because baseline levels of these hormones vary by sex. Additionally, there are unique reference ranges depending on your age and reproductive stage. For females, hormone biomarkers, like estradiol and follicle-stimulating hormone (FSH), are expected to be different in premenopausal women and postmenopausal women. For males, testosterone levels are known to decrease with age, but expected free T levels in certain age groups are not as well defined.

How do healthcare providers use reference ranges?

Reference ranges are an important tool for a health care provider reviewing your Everlywell at-home lab test results. They provide context to help your health care provider interpret the test results. Reference ranges are only a piece of the health care provider’s evaluation and decision-making process though; they are not clinical decision cut-offs.

Test results that lie outside the reference range do not necessarily mean there is a disease process that requires intervention; however, we always encourage you to follow-up with a health care provider to discuss your test results.

How can you use the reference ranges in your report?

Reference ranges are important to keep in mind if you have tested or plan to test with a different lab. For example, if Lab A has a reference range of 0.55-4.79 for thyroid-stimulating hormone (TSH) and your TSH level is 3.6, your test results from Lab A will indicate your TSH is “Normal”. However, if the same sample had been tested at Lab B who has a reference range of 0.5-3.0, and your test results are 3.2, your results would say your TSH is elevated. Both results are valid, but they are different because they are lab- and test-specific.

For these reasons, testing at the same lab can be helpful when tracking or comparing your results over time. This is not always possible though, and that’s why reference ranges are needed.

When the results interpretations for each test are different, it can also help to look at where you fell within each range. You may notice that the values are borderline (in other words, just below or just above the boundary). This is specific to reference ranges that are similar, where the biomarker measured is also the same.

TSH Results from Lab A TSH Lab A example

TSH Results from Lab B TSH Lab B example

You’ll see the reference range for each marker on the results dashboard or in the print-out version that you can share with a health care provider. For those interested in learning more about their results, Everlywell offers a complimentary live webinar and Q&A session with one of our health care professionals.


At the end of the day, we know variability in your test results can be alarming, but some variability between labs and within your own results over time is expected. It is recommended to follow-up with a health care provider if you notice significant, unexplained changes in your test results as this could signal a change in your health.

Have any specific questions about your test results? Please email our Customer Care Team.

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