How I can search searchable fields in SearchHelper.Search(parameters)

Hi! I have subCategoty column in my doc type wich I marked as "Searchable". How I can perform search by that field with base query? For example I put value "pdf" into "SearchFor" property and I want add "subcategory:44" expression. If I pass that to "SearchFor" prop it does not work.

Thanks!

SearchParameters parameters = new SearchParameters()
                {
                    SearchFor = "pdf SubCategory:44",
                    SearchSort = null,
                    Path = null,
                    ClassNames = null,
                    CurrentCulture = "###ALL###",
                    DefaultCulture = null,
                    CombineWithDefaultCulture = false,
                    CheckPermissions = false,
                    SearchInAttachments = false,
                    User = MembershipContext.AuthenticatedUser,
                    SearchIndexes = index.IndexName,
                    DisplayResults = 100,
                    NumberOfProcessedResults = 100,
                    NumberOfResults = 100,
                    AttachmentWhere = null,
                    AttachmentOrderBy = null,
                };

                // Search
                System.Data.DataSet results = SearchHelper.Search(parameters);