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 Help: Question on IC50 fitting		  New Topic  Reply to Topic
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alexxorigin

Hong Kong
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Posted - 01/02/2008 :  09:37:13 AM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic
Origin Version (Select Help-->About Origin): 7.0220(B220)
Operating System: Windows XP Professional

I have plotted a Dose-response curve and gonna fit with the IC50 function.
What I am trying to do is to fix the Hill's coefficient to be "1" and fit the curve. After fitting, the curve doesn't fit well to data points of the drug concentrations I administered. Yet, when I tried to vary the Hill's coefficient, the curve fit well to the data points.

The problem is that I want to fix the Hill's coefficient to be "1" in order to make comparisons of different curves. Is there any method I could do such that the curves can fit well with fixed Hill's Coef. to be "1" while at the same time the y-axis fit in the range between 0 and 1? Seemingly there's no way to set the y values but the x values instead in the IC50 function.

Please help and thanks a lot!

Edited by - alexxorigin on 01/02/2008 1:39:28 PM

Edited by - alexxorigin on 01/02/2008 1:40:17 PM

larry_lan

China
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Posted - 01/03/2008 :  01:59:48 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Hi Alex:

Could you please send me your OPJ to tech@originlab.com and then I can look into your problem.

Thanks
Larry
OriginLab Technical Services
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greg

USA
1379 Posts
Posted - 01/15/2008 :  1:49:43 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Fixing a parameter can constrain a function in a way that may make it impossible to get a good fit. In the case of a Hill function:
y = Vmax * x^n / (k^n + x^n)
fixing the Hill coefficient to 1 turns the whole equation into a hyperbola:
y = Vmax * x / (k + x)
Unless your data really is a hyperbola (not that common in dose-response since you'd like to know the low dose effect and not just the high ramp of high dose effect), you might try this in Origin 7:
Plot all the data you are interested in comparing.
Open the fitter and select the Hill function.
On the Action : Dataset page, click 'Fit Multiple Datasets'
Click 'Add Data' until you have enough variables.
Double-click the 'n' in Parameter Sharing to make it shared.
Assign the variables and proceed with the fit. While the Hill Coefficient won't be constrained to 1 (which you could still do by unchecking the 'vary' next to 'n') all the data fits will now have the same Hill Coefficient value.

In Origin 8, you can choose Global Fit on the Data Selection page and then check the Shared box next to 'n' on the Parameters tab. (Origin 8 is much easier in that it automatically includes all curves in the fit.)
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