Lab Tests: Normal range vs. optimal range

Normal Ranges

Normal ranges describe the set of levels of a marker (short for biomarker) that are associated with being healthy. They help physicians [R, R]:

Interpret patients’ lab test results
Make diagnoses
Decide on treatment

Normal ranges are also known as reference ranges or reference intervals.

In the context of lab test results, “normal” does not mean usual, typical, or ordinary. The word “normal” here means determined from a “normal distribution”.

Reference ranges are calculated from studies with a group of healthy people (reference population). When all the people’s test results are graphed, they look like a symmetrical, bell-shaped curve as shown below: a normal curve or distribution.

In a normal distribution, the mean value lies in the center. Half of the population will have values below this mean (left side); the other half will have values above (right side). The central set of values (in light gray), where 95% of a healthy population falls, is considered the normal lab range.

Once 95% of the values are accounted for, any remaining ones fall outside of the reference range. These are the ~2.5% tails on either end of the curve above. Any lab value that falls in these areas is flagged as high or low.

There are some exceptions to this. For example, for markers of heart damage called troponins, 99% of the healthy population have values that fall within the normal range. In that case, the 0.5% end tails are flagged as high or low [R].

Every lab marker is assigned a reference range. Some lab markers may have more than one reference range. This is due to factors that affect its levels. For example [R]:

Age
Sex
Reproductive status (puberty, menstrual cycle, stage of pregnancy, and menopause)
Race (e.g. prostate-specific antigen in African Americans)
Time of sample collection (e.g. random vs. first-morning spot urine sample)

Learn more about the reference ranges and their creation in this post.

Lab measurements also depend on that lab's equipment and methods of measurement. That is why each lab will recruit their own reference population and establish their reference ranges. It is also why reference ranges can differ slightly between labs.

In some cases, recruiting a reference population is expensive and time-intensive, so normal ranges may also be defined based on those established by other labs or test manufacturers [R].

We base our reference ranges on the normal ranges of the most widely used laboratories in the US, including Labcorp and Quest Diagnostics.

Optimal Ranges

Optimal ranges are the set of levels of a marker associated with better health outcomes. They consider general health, physical and mental performance, and longevity.

Reference ranges are values that the majority of a healthy population falls into. However, they may still be associated with an increased risk of mortality and certain diseases because they:

Are created using populations that may contain a significant number of potentially unhealthy individuals that don't get flagged as such at the time of measurement
Rarely take into account large population research looking into the risks for disease and mortality.

Optimal ranges are created to overcome these drawbacks. Therefore, they are within the normal range, but they do not include values that may be suboptimal.

At Selfdecode, we do a thorough literature review to determine whether there may be optimal ranges for all of our markers.

To determine optimal ranges for a lab marker, we look at three main things:

1) Does the marker cause a health condition?

2) Are there lifestyle interventions that can influence this marker AND prevent a health condition?

3) Is there a range within the normal range associated with better health outcomes, increased longevity, and better quality of life?

If the answer to all of the above is "yes", then we proceed to determine an optimal range for that lab marker.

To check for causality we look at [R, R]:

Large clinical trials
Mendelian randomization studies

Next, we look for meaningful lifestyle interventions (e.g. exercise, diet changes, supplements). They should help people get to optimal marker levels. Only then the optimal range is considered achievable.

Finally, we review association studies linking certain lab ranges to better health outcomes. We focus on longevity but also look at more specific outcomes such as heart disease or diabetes.

Following this process, we’ve obtained meaningful optimal ranges for many of our lab markers. They may help guide the person that is trying to get back to optimal health.

Read more about optimal ranges here.

Keep in mind

There is no universal reference range for most lab tests. Reference ranges may differ between laboratories and the methods they are using. Hence, you may get one number from one lab and a slightly different number for the same test from another lab. However, for the common tests, the difference between laboratories should be minor.

Just because your lab value is outside the normal range does not mean that you necessarily have a disease or disorder. Indeed, and by definition, 5% of healthy individuals will have levels outside of the normal range.

Conversely, a normal lab value does not guarantee the absence of a disease or disease process.

Our optimal ranges are based on extensive research. We check scientific studies focused on longevity and reduced incidence of disease. Due to the time-intensive nature of determining optimal ranges, not all labs offer them. Thereby, many healthcare providers may not be familiar with them.

Lab tests rarely mean much in and of themselves. They are just a piece of a puzzle your doctor is looking at. That is why your healthcare provider will examine your lab results in relation to one another. They will also take into account your:

Medical and family history
Any previous test results (to identify trends)
The reason for the test (routine health check, managing a disease, diagnosis)
Current medications

If you have any concerns about your test results, ask your healthcare provider for clarification or further testing.