Modern medicine still depends on this animal’s blood to test for bacteria in vaccines.

And an alternative test requires further study.

For decades, drug companies have depended on a component in the blood of the horseshoe crab to test injectable medicines, including vaccines, for dangerous bacterial contaminants called endotoxins.

Conservationists and some businesses have pushed for wide acceptance of an alternative test, to protect the horseshoe crabs and birds that feed on their eggs.

Earlier this year, these people seemed to be on the brink of success as the nongovernmental group that issues quality standards for such tests moved toward putting the alternative test on the same footing.

Pharmacopeia, announced that the alternative test known as rFC (recombinant factor C) requires significantly more study.

Pharmacopeia representatives said they have 30 years of data on the current test and only two years on the new test so they needed more information.

Internationally, the European Pharmacopeia has approved widespread use of the alternative test.

Her group supports technological solutions to conservation problems, including replacing the test that uses a component of horseshoe crab blood, called LAL, with a test that uses rFC.

It is the synthetic equivalent of a chemical in the crab blood, produced by inserting genes for its creation into microorganisms grown in the lab.

But companies that produce LAL from horseshoe crab blood say that the supply is adequate.

Phelan said this calculation “boggles the mind” because, “for every dose going out the door — each manufacturer will use 10 times the amount of LAL to test every step along the way in the process.” That includes vials, stoppers and other ingredients in the vaccines.

Phelan said there are likely to be numerous companies producing vaccines in the test phase and along the way.

If a company uses rFC, it must demonstrate to the F.D.A.

that, for each new drug, rFC is as effective as the standard LAL test.

One company that is turning to the new test in a big way is Eli Lilly, which also pushed for broader approval of the new test.

Jay Bolden, a biologist with Eli Lilly, said the company had been looking at rFC testing since 2015 for several reasons, including a consistent quality of a lab product, a supply that doesn’t depend on an animal population, a company commitment to replace animal use when possible and a reduction in costs.

“In 2016,” he said, “we kind of drew a line in the sand.” The company decided to use rFC for new injectable products even if additional work is required.

“It probably looks like a three-to-four year delay with no guarantee,” he said, that rFC would be considered equivalent to the LAL test.

Pharmacopeia, said that more evidence is needed to show the equivalence between the two tests because of the many years of data on LAL and a lack of data on rFC.

Bolden, at Eli Lilly, said that given the delays his company will continue using rFC but follow a different path in submissions to the F.D.A.

“We’ll just start using the European test chapter,” he said, referring to the European pharmacopeia’s inclusion of rFC, which “goes live July 1.” He said the company had reason to believe the F.D.A.