Hi Andy,

I'm new to this game as well and FLC are confusing because the normal ranges are so large. κ can be 3.3-19.4 mg/mL and λ can be 5.7- 26.3 mg/mL with the ratio (κ/λ) 0.26 -1.65. So you could theoretically have κ of 3.3 and λ of 26.3 giving a ratio of 0.125. Both would be in the normal range but the ratio would be way out from the norm. Similarly your ratio could be normal at 0.26 but your κ could be 19.4 and λ would be 74.6 - very high. It seems though that it is the change of ratio that is important. As your treatment progresses the ratio should return to the normal range if things go well, and much more quickly that other symptoms.

Say you presented with a κ of 100 mg/mL and a λ of 10mg/mL. Your ratio(κ/λ) is therefore 10. after a few months of chemotherapy your κ may be now 10 mg/mL and the ratio is now 1. This would be an excellent response. If the ratio is still 10 then they will try something else.

I think there is a bit of a wobble with the ratio as if the λ went to 12 (randomly) but no change in κ then you may think as the ratio is now 8.3 that things are improving but this is not the case, so the results sheet suggests only changes >20% are significant. There will also be imprecisions in the measurements, particularly in the lower range.

Some centres favour the dFLC (essentially the κ-λ or λ-κ), so in the example above this would be 100-10 = 90mg/mL. This should also get less during treatment.

I hope this helps. It has helped me writing it to better understand it. I also found a Youtube video by the manufacturers of the assay which I found helpful, primarily the first 16 minutes and especially 12-16 minutes.

https://www.youtube.com/watch?v=Ye9s6BRgx_Y