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What is a PPC? What is inhibition and enhancement?  

The name PPC or Positive Product Control is dropped regularly when discussing an endotoxin detection method. It is an essential part to demonstrate the validity of an endotoxin test. 

The LAL assay is a biological, enzymatical assay. It is based on either a change in turbidity (LAL-turbidimetric), a color difference (LAL chromogenic) or the formation of a clot (gel-clot). The key to these molecular reactions is the proper functioning of the test enzyme from the Limulus Polyphemus Lysate (LAL). Substances present in the test mixture or extract can affect the test enzyme reactivity. If reactivity is decreased, we would expect endotoxin to be underrepresented in the result (inhibition). Similarly, we would see endotoxin overrepresented if enzyme reactivity were increased (enhancement). These effects have the potential to result in either false negative or false positive test results.  

For example, let’s assume that a protein denaturing substance is present in a test sample. As the test enzyme is a protein it could potentially be destroyed before the completion of the assay. This would result in a reduction of reactivity, which would subsequently lead to a reduction of clotting, turbidity or colorization ultimately producing a detected endotoxin result that is less than the endotoxin present in the sample. Therefore, we might falsely assume that the sample does not contain endotoxins.  

To prevent reporting false negative or false positive results, at Nelson Labs, we test each sample with and without the addition of a known endotoxin spike of normally 0.5 EU/mL. If we subtract the endotoxin concentration from the unspiked sample from the endotoxin concentration of the spiked sample (Positive Product Control (PPC)) we should recover 50-200% of the spike. 

If the functioning of the enzyme is as it should, we will measure for an uncontaminated sample an endotoxin content of less than <0.005 EU/mL and for the PPC 0.505 EU/mL, assuming a recovery of 100%.  

A recovery of 50-200% might seem a large interval. However, this is due to the statistical phenomenon “A combination of Variances”. Without going into too much detail, this is a case where you subtract a number which is prone to an accepted level of variance from a number which is also prone to a certain level of variance. 

PPC’s should be included in every test method. As this is the only mean to demonstrate that the test system is reacting as expected and excludes the occurrence of false negative or positive results. It should be noted however that PPC’s do not exclude false positives caused by Beta-glucans, but this is a topic has been covered in our Beta-glucans post.  


Peter Cornelis, Ir.

Senior Expert Microbiology

Peter Cornelis graduated from the Catholic University of Leuven (Belgium) in 2000 as a Master in Applied Biological Sciences (Major Biotechnology). In 2003 he started working for Toxikon Europe (now Nelson Labs) as a study director Microbiology and in-vitro Toxicology. From 2007 until 2016, he was department supervisor for Microbiology and in-vitro Toxicology. Since 2016, he is responsible for research, validation, and development of new microbiological and in-vitro toxicological methods. Peter is a member of the ISO committee TC 194 WG5, Cytotoxicity and WG 8, Irritation and sensitization. As an expert, he was involved in the adaptation of ISO/TC 194 ISO/DTS 11796:2022(E) Biological evaluation of medical devices — Guidance for interlaboratory studies to demonstrate the applicability of validated in vitro methods to assess the skin sensitization of medical devices.

Nathan Pett

Study Director BET

Nathan Pett graduated from Utah State University in 2018 with a bachelors in Biochemistry. In 2019 he began work for Nelson Labs as a Lab Analyst and in 2021 transitioned to the role of Study Director in the BET department.

Emily Spackman, B.S., RM(NRCM)

BET Consulting Study Director

Emily Spackman has worked for Nelson Laboratories in the Bacterial Endotoxins Lab for over 16 years. She graduated from the University of Utah and holds a bachelors degree in Biology. She is a member of The National Registry of Certified Microbiologists and has experience with both medical devices and pharmaceutical products.