Laboratory Tests: U.S. Conventional units versus SI units and Their Conversion Factors

In pretty much all clinical trials, laboratory tests are performed as a tool for diagnosing the disease, assessing the safety / tolerability, assessing the pharmacokinetics / pharmacodynamics and so on. A very common issue is the reporting units for laboratory tests. There are two different unit systems: conventional units and SI units.

U.S. Conventional Units (may also be referred as United States customary units): In the United States, most people express distances in inches, feet, yards, or miles. Those units, along with the units we use for speed, volume, and other quantities, are known as the U.S. Conventional System.

SI Units or Système Internationale: The International System of Units (abbreviated SI from French: Le Système international d'unités) is the modern form of the metric system and is the world's most widely used system of measurement, used in both everyday commerce and science. Most scientists and most countries now use SI units. SI units use the meter, the kilogram, the second, and the kelvin. Each base unit measures a different quantity. For example, the meter measures length, and the kilogram measures mass.

The units of these two systems are different, but the quantities they represent do not change. The units have a fixed relationship to each other. The laboratory results reported in U.S. Conventional Units can be converted into the results in SI Units or vice versa. The relationship to convert a value from one system to the other is called conversion factor. Below are some of the websites containing the conversion factors for common laboratory tests. The SI Conversion Calculator or conversion factor table provided by JAMA Author Instructions may be the most common one that has been used.

• SI Conversion Calculator
• Clinical Analyte Unit Conversion 
• SI Units for Clinical DataNormal Laboratory Values from AIDS.org
• AMA manual of style laboratory unit table 
• Conventional Units - International UnitsClinical Chemistry SI Conversion Factors 

Due to the differences between two unit systems, for a clinical trial where the central laboratory services are used, the decision needs to be made on which unit system is used for reporting the laboratory results to the investigators and to the sponsors. If it is a domestic trial in U.S., it is better to report the central laboratory test results in U.S. conventional units to the investigators. If it is multinational trial, it is better to report the central laboratory test results in S.I. units. For laboratory test results transferred to the sponsor, the results in both units can be requested.

The following may also be useful in understanding the different laboratory test units:

Factor Prefix Symbol for Lab Unit
10^12 tetra T
10^9 giga G
10^6 mega M
10^3 kilo k
10^(-3) milli m
10^(-6) micro µ
10^(-9) nano n
10^(-12) pico p
10^(-15) femto f
10^(-18) atto a

1 mL = 1 cc where mL is milliliter and cc stands for cubic centimeter
mg = milli gram = one-thousandth of a gram. 1 g = 1000 mg
1 mg = 1000 mcg or ug, here mc stands for "micro-" meaning "one millionth of".
In cell counts, the results may be reported as #/cumm, here cumm = mm^3 meaning per cubic millimeter.

For microbial measures (such as viral counts and viral loads), the log scale is commonly used. Microbial load (cfu/g or cfu/ml) can be expressed as log10. So, if you have 100,000 microbes that is 5 log, 10,000 microbes is 4 log, 1,000 is 3 log, 100 microbes is 2 log and 10 microbes is 1 log. Now, if you went from 100,000 microbes cfu/g to 10,000 microbes cfu/g that would be a 1 log reduction (5 - 4 log). If you went from 100,000 to 32,000 that would be a 0.5 log reduction (5 - 4.5 log) and so on.
If the microbial population went from 100,000 to 32,000 that would be a 0.5 log reduction (5 log - 4.5 log).
If the microbial population went from 100,000 to 320,000 that would be a 0.5 log increase (5.5 log - 5.0 log)