Third-party testing means a laboratory independent of both the manufacturer and the seller analyses a sample and issues its own report. The distinction from an in-house certificate of analysis is one of incentive, not of technique: the same HPLC and mass spectrometry methods are used, but the party running them has no commercial interest in the outcome.
What it verifies, and what it does not
A meaningful independent report confirms identity by exact mass against the theoretical molecular weight, chromatographic purity as an area-percent figure with the chromatogram attached, and — where commissioned — net peptide content, water and counter-ion content, sterility or endotoxin.
Its limits are equally definite. A report covers one sample from one lot on one date; it says nothing about the next batch. Area-percent purity by UV at 214 nm measures peptide-bond-containing species and can miss non-absorbing contaminants entirely, and residual solvents, heavy metals and bacterial content are separate assays that must be ordered explicitly.
Verify the report itself: named laboratory, analysis date, a lot number matching the vial, and an attached chromatogram rather than a summary table. A PDF without those elements is a claim, not evidence.
Related terms and material
purity · gray-market vendor · FDA-approved vs research-grade. Further reading: third-party testing explained, how to read a peptide COA and verifying a lot yourself.