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Biological Indicator – An Industry-Sparking Innovation

Andersen Sterilizers founder, H.W. Andersen, M.D., designed and patented the first self-contained Biological Indicator (BI) in 1969. Long after his patent expired, his invention has become an industry in and of itself.

“Biological Indicators—then and now—are the only method approved by the FDA for releasing devices after a sterilization cycle for use in human healthcare,” said Dr. Andersen’s son William Andersen, M.D., current President of Andersen Sterilizers. “However, before my dad’s invention, devices were released after incubating spore strips, a process fraught with potential cross-contamination.”

At the end of a cycle, a tech would cut open a glassine envelope containing a spore strip, remove the Bacillus subtilis strip with sterile forceps, deposit the strip in a test tube filled with a nutrient broth and incubate it for at least 48 hours in a warm place. As one can imagine, there are many points of possible contamination in this transfer process that could lead to false positives.

On the other hand, Dr. Andersen’s self-contained BI, which he called “Steritest,” packaged a glass ampoule filled with growth medium and a spore dot in a pouch comprised of clear plastic and Tyvek. The medium contained a pH change dye. After a cycle, the tech would break the ampoule inside the pouch, shake the pouch to coat the spore dot, and let it sit in a warm place for 48 hours.

Inside the hermetically sealed pouch, there was no possibility of outside contamination. If any of the spores survived the cycle, the living organisms changed the pH and therefore the color of the nutrient broth, indicating a failed cycle.

“Dr. Andersen’s particular brand of genius,” said A.E. “Ted” May, Andersen Products CEO, “is to bring to market very simple engineering solutions to nagging everyday problems.”

Dr. Andersen added a Dosimeter to each Steritest, within the same plastic and Tyvek pouch, as an initial “screening test” to confirm adequate time, temperature and gas concentration during the cycle.

Since that time, other manufactures have built on Dr. Andersen’s invention, creating an extremely compact self-contained BI that houses the nutrient plug and filter paper embedded with B. athophaeus spores inside of a thermoplastic culture tube. The tube is capped with a Tyvek portal and displays a color change indicator label.

When Dr. Andersen’s patent expired after 17 years, he believed companies whose sole focus was self-contained BIs could provide a more efficient means for producing the devices. Today, there are multiple companies entirely devoted to the manufacture of self-contained BIs. The BIs are made in vast quantities and are therefore more economical for buyers—something that has always been important to Dr. Andersen.

Dr. Andersen’s more than 80 patents, many of which sparked entire industries, will be one of his most powerful and enduring legacies.

Steritest BI Patent
The first page of Dr. Andersen’s 1969 Steritest patent – the first self-contained Biological Indicator.
Andersen's Steritest - the first self-contained Biological Indicator - front and back view.
Andersen’s Steritest, front and back view.