The trade association initiated the TruLabel program in 1990 to promote quality assurance of dietary supplements.
The current test was conducted largely to counter accusations made against the dietary supplement industry in the instances where athletes have pointed the finger at their supplements when caught in a doping scandal.
"We're not looking to be a drug-testing organization," said Daniel Fabricant, NPA vice president of scientific affairs.
"But we're trying to allay some of the fears that are out there."
NPA said it randomly selected the eight supplements from mainstream products sold through retailers and the Internet.
However, it did indicate that these were association members' products.
The association tested the sports supplements using gas chromatography mass spectrometry.
The turn around time for the testing was one week, and NPA plans to follow-up with different labs and keep them blinded.
"This is a way of putting our money where our mouth is," said Fabricant.
In a market what sports nutrition is big business, sports supplements in particular have taken a bad rap.
"The perception is that sports drinks aren't a problem and sports supplements are," said Fabricant.
"But really they are both designed to replace nutrients and energy sources that are lost during exercise."
NPA said there are various reasons why sports supplements have taken the blame in sports doping scandals.
Firstly, the allegations a supplement contained stimulants may have been false.
Secondly, the products could be steroids masquerading as stimulants.
NPA would now like to expand TruLabel into testing non-member products as well, though it will not aim to make the program and its results official.
"We're not looking to get certified," said Fabricant.
The NPA tested were from The Vitamin Shoppe, Nature's Best, Jarrow Formula, Wellements, Vital Pharmaceuticals, Now Foods, Phytoceutical Formulations and Ultimate Nutrition.