Some samples can severely interfere with the biochemical reactions involved in the Limulus Amebocyte Lysate (LAL) test, and may even prevent the reaction from occurring. In such cases, special sample pre-treatment procedures may be required before endotoxin testing.
Sample Pre-treatment Methods
Dilution Method
A considerable number of interfering substances exert their effects only at certain concentrations. Appropriate dilution of the sample can therefore reduce interference to an acceptable level, allowing the sample to be tested using the conventional procedure. Examples include injections containing mannitol, gentamicin, human serum albumin, sodium heparin, and vitamin C.
Heat Treatment
Some interfering substances are heat-sensitive. Holding the sample at an appropriate temperature for a specified period of time may eliminate or reduce their interference, after which the sample can be tested using the conventional procedure. This approach may be applicable to certain blood products and biological products.
pH Adjustment
Some samples have excessively high or low pH values that cannot be brought into the appropriate range by the buffering system in the LAL reagent. In such cases, the sample pH may be adjusted using appropriately prepared hydrochloric acid or sodium hydroxide solutions that have been confirmed to be sterile and pyrogen-free. The sample can then be tested using the conventional procedure. Amino acid injections are one example.
Ultrafiltration–Back-Flushing Method
Traditional Chinese medicine injections generally contain complex mixtures and multiple interfering substances, making them difficult to treat using conventional approaches. In such cases, ultrafiltration may be used. The sample is passed through an ultrafiltration membrane, while endotoxin is retained on the membrane. The filter and membrane are then rinsed with pyrogen-free water. After reversing and securing the membrane, pyrogen-free water is used to back-flush the endotoxin into a volumetric flask. The volume is adjusted to the specified mark, and the resulting rinse solution is tested using the conventional procedure.
Solid Samples
If the solid sample is soluble, it should be dissolved in pyrogen-free water and subjected to an inhibition/enhancement test using the methods described above.
For solid medical devices such as surgical instruments and disposable infusion sets, an appropriate amount of pyrogen-free water can be used for soaking or rinsing according to the applicable procedure. Because endotoxin is relatively water-soluble, endotoxin present on the internal or external surfaces of the device can be released into the liquid after appropriate soaking or rinsing.
Important: Recovery Test
For complex samples, a combination of the above pre-treatment methods may be required. Whether one method is used alone or two or three methods are combined, inhibition or enhancement may occur during sample processing. Such interference can cause the measured endotoxin concentration to be either higher or lower than the actual concentration. Therefore, a recovery test must be performed when applicable, and the actual endotoxin concentration should be calculated based on the measured recovery.