I was reading alot about this problem. Mostly old topics. It s almost 2016 is there any solutions to this.

My art have some filters and gradients on it , so today I was trying to get output PDF in CYMK colors using Scribus , but finished file don t have filters and gradients and part of the picture that I clipped prevously in Inkscape is all over the place. Real mess. I need this art for printing on shirts. So any help is welcome. Thank you.

Unfortunately, Scribus does not support all SVG features that Inkscape supports, especially filters, clips/masks and gradients are difficult, in my experience.
You can replace the clips by real path operations (cut the paths), and edit the gradients inside Scribus to get them back (I also tend to loose them... seems to be a problem with where the gradient stops end up). Filters could be rasterized, depending on their location.

Another option would be to export to png in a very high resolution and convert to CMYK with a raster image editor.

Or the printing service may accept RGB files, too, and convert them themselves. In this case, you should make sure you get a proof before they really start printing.

Remember that PDF/X-3 and X-1(a) don't support any transparency at all, which may also affect filters and gradients in your output.

Or you might want to try the extension in this topic: viewtopic.php?f=11&t=5943

Does it still work? Would be interesting to know (I think I read somewhere that it doesn't...)

Edit:
It doesn't.
First of all, the .inx file is defect (I was able to fix that by removing an extra > and adding in the submenu where it is supposed to live) - and then it doesn't know the id attribute or is missing a file (depending on whether you use pdf or tiff export). The Tiff thing even closes the whole Inkscape application - oh, but it creates a CMYK tif file before it crashes. Can't find a mention of the color profile I used, though, in the output of 'identify', in contrast to other CMYK tiffs I have.
Can't recommend :/

Thank you guys on quick reply. Well I will try that with high resolution PNG.
Man I love working in Inkscape, but it looks like it s still same story.Inkscape is more for web. If you want to do something serious you need AI. I wonder will they ever find solution to this.

Oh no, I wasn't aware that extension didn't work. I think it did originally -- not sure why author doesn't maintain it.

OH! But they are still working on something. From later in that thread:

jonata wrote:All of these extensions aren being developed anymore and are being replaced by a new and more flexible extension called "Inkscape OUTPUT PRO". You can see more details here:
http://jonata.org/inkscape/outputpro/ (only in portuguese).

Thanks!

Edit
Starting here: viewtopic.php?f=11&t=5943&start=25#p45043 a good deal of discussion on OutputPRO, including English translation.

Wait, is this the same thing? http://wiki.inkscape.org/wiki/index.php/ExportPDFCMYK Or is this a completely separate extension? According to: viewtopic.php?f=11&t=5943&start=25#p45028, different extension (iiuc) but doesn't work either.

(Just to have all this related info in 1 place )

Outputpro never left the jpg stage, unfortunately.
And - judging from the screenshots - yes, the extension in the Wiki is the same one.

@Cashville: yes, people have been asking for this for years, but it seems to be a difficult thing to implement, or doesn't attract enough interest than fun new features and severe crashes do.
Maybe with the funded development (https://inkscape.org/en/support-us/funded-development/), there will be a way. Or if someone who's really involved in print workflow becomes interested in development.
It's not impossible to do this with command line tools - only difficult for the average user... and results may not always be convincing.