Everything you need to know
There is no single endotoxin limit that applies to every peptide. Under USP <85> the limit for an injectable is calculated from the dose, so the same endotoxin result can pass at one dose and fail at another. This page shows the calculation.
What is the endotoxin limit for a peptide?
For an injectable, the compendial limit is K divided by M, where K is 5 Endotoxin Units per kilogram of body weight per hour and M is the maximum dose per kilogram given in one hour. For a 70 kg adult that is a ceiling of 350 EU from everything injected within an hour. The limit is therefore a property of the dose, not of the peptide. For the intrathecal route K is 0.2 EU per kilogram, which is 25 times stricter. Research peptides sold for laboratory use have no regulatory endotoxin limit of their own; the K over M calculation is the standard that applies to injectable drug products, and it is the sensible reference point.
How do I calculate the limit in EU per mg?
Divide 5 EU/kg by the dose in mg per kg. Example: a 2 mg dose in a 70 kg adult is 0.0286 mg/kg, so the limit is 5 divided by 0.0286, which is 175 EU per mg. A 10 mg dose in the same person is 0.143 mg/kg, giving 35 EU per mg. The larger the dose, the lower the allowed endotoxin per milligram, because the total delivered must stay under 350 EU for that 70 kg adult. If a certificate reports 2 EU/mg, a 2 mg dose delivers 4 EU and a 10 mg dose delivers 20 EU, both far below the ceiling.
Does the reconstitution water count?
Yes. The limit applies to everything injected, so endotoxin in the water adds to endotoxin in the peptide. The USP monograph limits are 0.25 EU per mL for Sterile Water for Injection and 0.5 EU per mL for Bacteriostatic Water for Injection. Water from an unverified source can carry more endotoxin than the peptide does, which is why a water result is worth having alongside the vial result. The reconstitution water panel reports endotoxin, benzyl alcohol and pH.
A vendor certificate says below 5 EU/mg. Is that a pass?
It is a specification the seller chose, not a compendial limit, and whether it is safe depends on the dose. Use the calculation above with your own numbers. Also check three things on the certificate: the method (LAL or recombinant factor C under USP <85>), the actual result with units, and the limit it was compared against. A bare pass with no number and no stated limit cannot be checked.
Why can a low result still be wrong?
Two reasons. Some peptides interfere with the lysate reaction and suppress the signal, so a valid test includes a positive product control showing that endotoxin added to the sample is recovered; without it, a low number may only mean the test was inhibited. And endotoxin is easily introduced during sampling, so the aliquot must be handled with endotoxin-free consumables from the first transfer. Ask whether the result passed its spike recovery check.
How is endotoxin reported at Prodigy Labs?
By kinetic turbidimetric LAL under USP <85>, as a number in EU/mL and EU/mg. We report the measured value. If you give us a specification limit, we report the result against it and state that limit on the certificate. We do not calculate a limit from a dose, because the dose is yours to decide; the worked example above shows how to apply your own numbers. Endotoxin is $175 on its own and is included in both injectable panels. This page explains a laboratory calculation and is not medical advice. More on our endotoxin testing.