"The SF biomarkers of ADAMTS-5, CTX-II, C2C and IL-b had...

"The SF biomarkers of ADAMTS-5, CTX-II, C2C and IL-b had insufficient numbers of results above the lower limit of quantification (LLQ), therefore the data for these biomarker data could not be reliably interpreted."

I didn't see the poster before, but this seems to be in line with my explanation that either they couldn't really detect these proteins in enough of the synovial fluid samples to generate meaningful results or they were only present in very low quantities. To understand this, we first need to understand that synovial fluid contains a very large protein called hyaluronan and also proteins from the blood i.e. plasma proteins, as well as proteins secreted from the joint area. You are also restricted to obtaining only milliliters of synovial fluid, in contrast to being able to collect liters of urine (as pointed out at the end). If I recall correctly, they used LC-MS/MS to quantify these proteins.

LC MS/MS involves taking the synovial fluid, which can be extremely viscous i.e. forms clogs, and treat it with a protease i.e. an enzyme that cuts up other proteins into a predictable peptide mixture, with each peptide representing its original parent protein. So essentially if I wanted to quantify CTX-II, I would only look for a peptide fragment that is specific to CTX-II.

LC refers to liquid chromatography which separates the peptide mixture based on certain biochemical characteristics of each peptide. You cannot see everything in the peptide mixture, only some of it (around 10-30%), and whether of not you can see the peptide that corresponds to CTX-II is dependent on many factors.

MS/MS refers to mass spectrometry, which is the method used to detect and quantify. Recall how LC is used to separate the mixture. MS/MS generates a signal-to-noise ratio for each peptide it detects and it is normally the area under the curve that you quantify. If the signal is within the noise then you cannot reliably quantify it, which looks to be the case here.

Urine is much easier to analyse since you just isolate the proteins and process it as above. You can even take larger samples of urine and concentrate the protein, thereby overcoming the issues associated with SF. Serum is blood minus the plasma proteins and similarly to urine is easier to analyse then SF.