Reference range values are for apparently healthy individuals. Every laboratory should have developed its own reference ranges for all of the procedures it performs. While all laboratories' ranges will be close, there will be variations due to collection, storage, transport, preparation techniques, types of instruments used, and the specific patients in the laboratory's population.

Reference ranges are calculated by performing the same test on a number of "assumed healthy" people that mirror the age or gender of the population that is of interest. The greater the number of individuals in the pool, the more valuable the range will be. Once the results are finalized, an average (mean) is calculated. The next step is similar to a teacher putting a class's grades onto a Normal (Gaussian) Curve. The calculation is called a standard deviation. The range of +/-2 standard deviations should be wide enough that 95% of all of the test results should be included. This is the reference range.

Since the ranges that are developed are averages, not a definition of "normal", the best way to look at these values is as a reference-something with which to compare yourself against others in your situation. The best comparison, however, is against your own previous reports.

In the US, the reference range for total white blood cell counts is 4,000 to 11,000 or 4.0 - 11.0 x 10 raised to the 9^th power per liter or 4.0 - 11.0 x 10 raised to the 3^rd power per microliter. The various types of white blood cells are often expressed as a percentage of the total white blood cell count. Usual percentage ranges are as follows:

These percentages are derived from 1) a microscopic examination of blood performed manually in which some hundreds of cells are differentiated from each other (this procedure is called a differential or diff) or 2) a machine scored differentiation based on cell patterns.

Absolute counts are calculated by multiplying the total white blood cell count by the percent of the specific cell line in which you are interested. A percentage of 50% neutrophils in a total white blood cell count of 6,000 equals an absolute neutrophil count of 3,000. While percentage reports are considered adequate for most patients, absolute values are more important for patients with hematologic disorders.

In order to minimize variations based on laboratory differences, CLL patients who are tracking their counts may want to always use the same lab for their blood count evaluations. It's important to remember that these statistics work only on populations, not on individuals. You need to compare yourself to you, not a scale.