Bioequivalence and Bioavailability Forum

Dear All,

According to the literatures, everybody have citicoline/uridine in blood. Therefore to determine the drug's plasma concentration of citicoline/uridine a baseline correction have to be done. Before dosing, we took 6 sampling points to determine citicoline/uridine level for the subjects individually for each period. We took geometric mean for these sampling points to find a baseline for this subject's citicoline/uridine level without drug effect.

After dosing, we subtracted the baseline value for all of the measured plasma concentrations to find the only drug's effect as concentration values.

For example:

Measured concentrations
Sampling time(h): Concentration (ng/mL)

t-48h:                   196.899
t-24h:                   158.358
t-22h:                   291.401
t-20h:                   343.588
t-18:                    434.975
t0h:                     342.337
t0.25:                   218.036
t1.00:                   999.380
t2.00:                  1457.960
t4.00:                   972.150

Corrected concentrations:
Before dosing geometric mean of t-48h to t0h (baseline value): 328.128
t0.25h: 218.036-328.128=-110.092
t1.00h:                  671.252
t2.00h:                 1129.833
t4.00h:                  644.022

As far everything is looking fine except negative value. In each batch, to calculate the correct amount of the Standards (ST) for calibration curve (QC) and Quality Control samples, the calibration curve plotted beginning with Zero plasma. Therefore the blank plasma sample's amount which have citicoline/uridine is almost 0 ng/ml. In other words, for STs and QCs a baseline correction already have been done in amount calculation via areas. The STs and QCs have been prepared with the sample plasma pool in all batches (not subject's blank plasma)to minimize the citicoline/uridine plasma deviations for each plasma.

My problem is this:
In the calculation of amounts in areas with the calibration curve, the curve starts with the zero plasma (but have citicoline/uridine for example 400 ng/ml).

It calculates this citicoline/uridine plasma level (400 ng/ml) as 0 ng/ml for zero plasma. If the subject's plasma concentration level is below that zero plasma concentration level (i.e. 250 ng/ml), the amount will be a negative value (-150 ng/ml) which Shimadzu Labsolution software calculates negative amounts.

With negative values (and baseline corrected values), I cannot calculate geometric mean to find baseline level for this subject. If I knew the zero plasma level's real amount (in this example 400 ng/ml), I add this value to all subject concentrations to find real amounts (in this example -150+400 = 250 ng/ml)

What do you recommend for this problem?

Thanks in advance.

Erkin Alkan