Blood Drug Testing

Why do the "normal" ranges of various blood tests differ between labs?

I received the results of a blood test today. Looking online I see that while my lab placed some of them in the normal range, they fall in the low range on all of the sites I've checked. Likewise one test came back from my lab high - and it shows in the normal range online. How does one determine which range to use?

Public Comments

1. The normal range for a lab is determined by a statistical analysis of the patient population being tested in that lab. A subset of the Food and Drug Administration called CLIA requires laboratories to regularly collect data from the patient populations they test regularly. The normal ranges are drawn from these samples and change as the poulation changes. Because each lab (in the US) serves a different region and patient population, the demographics (age, gender, ethnicity, risk factors, disease prevelance...) differs from lab to lab. As a result, the normal ranges differ slightly from lab to lab. If you look at the US population as a whole, the averages usually overlap. EDIT: Sorry. Missed your second question. Always use the reference range from the lab that performed the test in question. Do not compare reference ranges between labs.
2. The WNL (Within-Normal-Limit) Values provided by the lab that ran the test. Different labs will have different WNL Values. This is because: 1. Lab testing methods differ. For example, two labs may be running a similar test but with different reagents (chemicals) or on different instruments. 2. Lab calculation methods differ. An easy example is with a lab value referred to as Anion Gap. You will find quite a wide range of WNLs for Anion Gap if you did a search. But thats partly because calculations differ (some labs calculate Potassium Ion levels into the equation, but since they are relatively small some labs will leave them out). 3. Local regions have different WNL values. You know how it takes a cake less time to back if you are at high altitude compared to low altitude (or vice versa, I forget)? Well human physiology is also different at different altitudes. So a lab working out of Denver Colorado (6000 feet above sea level) doing local blood gas calculations may have slightly different blood gas WNL values as a lab in Death Valley, California (300 feet below sea level). 4. Demographics have different WNL values. I could go on a while, but at the risk of offending some overly sensitive person, so I'll let you think about it. Just imagine a lab reporting values in China as opposed to the US. Certain lab results might be slightly different, at least in some aspects.