I have this interpolation example

zzz = Interpolation[{     {{1}, 2`100*Pi},     {{2}, 3`100*Pi},     {{3}, 4`100*Pi}     }, InterpolationOrder -> 1]; zzz[2] zzz[2.5] zzz[2.5`100] 

and I am getting these resilts

9.424777960769379715387930149838508652591508198125317462924833776923449218858626995884104476026351204  10.9956  10.99557428756427633461925184147826009469009289781287037341230607307735742200173149519812188869740974 

first number as expected with 100 digits, second with only 4 digits, but if I add `100 to the argument I can get it with 100 digits as well.

Now I use multidimensional example

testData = {    {{-2.*10^-6, 0, -18}, -1.00853429340956742965680093076966025423734590973983188199800080460508043197286933569747093680631648*10^6},    {{-2.*10^-6, 0, -17},      952504.61044236930112440429386078472672512393090882097358420180063200222870428099604804394164585046},    {{-2.*10^-6, 1, -18}, -1.008544941816832295575440188033483155333800668013690157106923425838109768669333223372232072904210136*10^6},    {{-2.*10^-6, 1, -17}, -952514.6672734985329986920711230513627204920640531655413765067694156739368659196153982113188871999717},     {{-1.78947*10^-6, 0, -18}, -1.00854109353837536345550608172429712197251598047769246281925070613484574642246277456744911673832572*10^6},    {{-1.78947*10^-6, 0, -17}, -952511.41057117723492310944481542159446029400164668155440545170216176754315387443491802212157785970},     {{-1.78947*10^-6, 1, -18}, -1.008551741385092646935524440345434727928230066236270646432464156954808417875408736480254883876917917*10^6},    {{-1.78947*10^-6, 1, -17}, -952521.4668417583449142805833940440259658101867440498419481296004930320450422321944211465035742782570}    }; ff[x1_, x2_, x3_] = Interpolation[testData, InterpolationOrder -> 1][x1, x2, x3] 

and for this example, I cannot get the results with 100 digits, even if I use exact existing values

ff[-2.0`100*10`100^-6, 0.0`100, -18.0`100] ff[-2.0`100*10`100^-6, 1.0`100, -18.0`100]  Out[350]= -1.00853*10^6 Out[351]= -1.00854*10^6 

How to force interpolation to use required precision? There is no WorkingPrecision argument for interpolation.