U.S. Pharmacopeia OKs Synthetic Alternatives to Horseshoe Crab Blood

As of May 1, 2025, the U.S. Pharmacopeia, which helps to ensure drug quality in the United States and worldwide, has officially leveled the playing field between horseshoe crab blood and synthetic alternatives to test for endotoxins ­—an advancement only possible because of an MBL discovery decades ago.

In the 1960s, pathobiologist Frederick Bang was a visiting researcher at the MBL. While studying the circulatory system of the horseshoe crab, he accidentally discovered that a certain bacteria caused the crab blood to clot. He enlisted the help of hematologist Jack Levin, who identified the culprit as an endotoxin, which is a component of some bacteria that can cause shock, fever, and sepsis.

Levin used the horseshoe crab blood to develop the Limulus amebocyte lysate (LAL) test, which can be effectively used to screen drugs, intravenous fluids, and medical devices for bacterial contamination, ensuring their safety before administration to patients.

To obtain LAL, horseshoe crabs are bled in a lab and released back into the wild. Hundreds of thousands of horseshoe crabs are bled each year, and LAL testing saves many human lives, but not all the horseshoe crabs survive after being bled. Conservation groups estimate the mortality rate of released crabs to be at least 15% to 30%. This biomedical harvesting, along with habitat destruction and collection of horseshoe crabs for eel and whelk bait, has led to marked decline in the horseshoe crab population, which impacts other species like migratory shorebirds who rely on horseshoe crab eggs for food.

Horseshoe crabs are not only studied for their blood. In 1967, Haldan Keffer Hartline earned a Nobel Prize for his discovery that visual cells send signals through nerve cells to the brain. He used a horseshoe crab as an experimental model because of its compound eye and easy-to-isolate optic nerve.

At the MBL, researchers study horseshoe crabs to learn about genes that control eye development during embryogenesis.

As far as endotoxin testing goes, however, there is now an alternative to bleeding the horseshoe crabs. Over the past few decades, researchers have developed synthetic alternatives, recombinant proteins that are based on the key components in LAL, that can be used as an effective alternative. Research shows they are just as effective and will reduce the need to bleed horseshoe crabs by as much as 90%.

Last week, the US Pharmacopeia, which regulates drug quality in the U.S., officially approved chapter 86, which puts recombinant endotoxin tests on the same level of acceptability as the biological LAL reagent.

Now, researchers can use the synthetic Recombinant factor C and recombinant cascade reagent to test pharmaceuticals for endotoxins without the use of horseshoe crab blood. This is a necessary step in helping the horseshoe population recover, but there still lies a long road ahead.