Breath test data modeling

Dear all, We evaluate gastric emptying with a 13C-spirulina breath test, which was validated against the gold standard (i.e., scintigraphy). We are trying to apply the Wagner Nelson equation to analyze the 13C breath excretion curves. However, in patients with delayed gastric emptying, the absorption of 13C, hence peak breath excretion, has not occurred by them. Therefore, we do not have a reliable breath elimination constant, which is in the equation, in some patients with delayed gastric emptying. Collecting breath samples for a longer duration is not practical So we are trying to use mixed effect modelling to estimate Kel in patient with delayed gastric emptying, however plasma observations are not available in our data and we only have the rate of CO2 excreted in breath at each breath sample and the amount of 13C in the meal Any suggestions how to model this without having plasma observation in our data? The Wagner Nelson formula is written as: F[t] = (Abreath(t) + C(t)/Kel)/Abreath(∞) where F[t] is a fractional dose of the carbon label emptied, C(t) is the 13CO2 excretion rate (%dose ⁄ h) Thanks Ramy Abdelrahman, MBBCh Clinical pharmacology fellow Mayo clinic [email protected]<mailto:[email protected]>