diff --git a/test/derivative_tests.jl b/test/derivative_tests.jl
index 484c0330..d3abbdd2 100644
--- a/test/derivative_tests.jl
+++ b/test/derivative_tests.jl
@@ -1,4 +1,5 @@
 using DataInterpolations, Test
+using FindFirstFunctions: searchsortedfirstcorrelated
 using FiniteDifferences
 using DataInterpolations: derivative
 using Symbolics
@@ -35,6 +36,11 @@ function test_derivatives(method, u, t; args = [], kwargs = [], name::String)
             adiff2 = derivative(func, _t, 2)
             @test isapprox(fdiff, adiff, atol = 1e-8)
             @test isapprox(fdiff2, adiff2, atol = 1e-8)
+            # Cached index
+            if hasproperty(func, :idx_prev)
+                @test abs(func.idx_prev[] -
+                          searchsortedfirstcorrelated(func.t, _t, func.idx_prev[])) <= 1
+            end
         end
 
         # t = t0
diff --git a/test/interpolation_tests.jl b/test/interpolation_tests.jl
index 24811680..99c849e3 100644
--- a/test/interpolation_tests.jl
+++ b/test/interpolation_tests.jl
@@ -1,7 +1,18 @@
 using DataInterpolations
+using FindFirstFunctions: searchsortedfirstcorrelated
 using StableRNGs
 using Optim, ForwardDiff
 
+function test_cached_index(A)
+    for t in range(first(A.t), last(A.t); length = 2 * length(A.t) - 1)
+        A(t)
+        if hasproperty(A, :idx_prev)
+            @test abs(A.idx_prev[] -
+                      searchsortedfirstcorrelated(A.t, t, A.idx_prev[])) <= 2
+        end
+    end
+end
+
 @testset "Linear Interpolation" begin
     for t in (1.0:10.0, 1.0collect(1:10))
         u = 2.0collect(1:10)
@@ -45,6 +56,7 @@ using Optim, ForwardDiff
     @test A(0) == [-2.0 0.0; -3.0 0.0; -4.0 0.0]
     @test A(3) == [4.0 6.0; 6.0 9.0; 8.0 12.0]
     @test A(5) == [8.0 10.0; 12.0 15.0; 16.0 20.0]
+    test_cached_index(A)
 
     # with NaNs (#113)
     u = [NaN, 1.0, 2.0, 3.0]
@@ -168,6 +180,7 @@ end
     @test A(2.5) == 6.25
     @test A(3.5) == 12.25
     @test A(5.0) == 25
+    test_cached_index(A)
 
     # backward-looking interpolation
     u = [1.0, 4.0, 9.0, 16.0]
@@ -182,6 +195,7 @@ end
     @test A(2.5) == 6.25
     @test A(3.5) == 12.25
     @test A(5.0) == 25
+    test_cached_index(A)
 
     # Test both forward and backward-looking quadratic interpolation
     u = [1.0, 4.5, 6.0, 2.0]
@@ -204,6 +218,9 @@ end
     @test A_f(3.5) ≈ l₂ * u[2] + l₁ * u[3] + l₀ * u[4]
     @test A_b(3.5) ≈ l₂ * u[2] + l₁ * u[3] + l₀ * u[4]
 
+    test_cached_index(A_f)
+    test_cached_index(A_b)
+
     # Matrix interpolation test
     u = [1.0 4.0 9.0 16.0; 1.0 4.0 9.0 16.0]
     A = QuadraticInterpolation(u, t; extrapolate = true)
@@ -321,6 +338,7 @@ end
     @test A(8.6) ≈ 4.1796554159017080820603951
     @test A(9.9) ≈ 3.4110386597938129327189927
     @test A(10.0) ≈ 3.0
+    test_cached_index(A)
 
     # Test extrapolation
     A = AkimaInterpolation(u, t; extrapolate = true)
@@ -345,6 +363,7 @@ end
         @test A(3.5) == u[1]
         @test A(4.0) == u[1]
         @test A(4.5) == u[1]
+        test_cached_index(A)
 
         A = ConstantInterpolation(u, t; extrapolate = true) # dir=:left is default
         @test A(0.5) == u[1]
@@ -356,6 +375,7 @@ end
         @test A(3.5) == u[3]
         @test A(4.0) == u[1]
         @test A(4.5) == u[1]
+        test_cached_index(A)
     end
 
     @testset "Matrix case" for u in [
@@ -372,6 +392,7 @@ end
         @test A(3.5) == u[:, 1]
         @test A(4.0) == u[:, 1]
         @test A(4.5) == u[:, 1]
+        test_cached_index(A)
 
         A = ConstantInterpolation(u, t; extrapolate = true) # dir=:left is default
         @test A(0.5) == u[:, 1]
@@ -383,6 +404,7 @@ end
         @test A(3.5) == u[:, 3]
         @test A(4.0) == u[:, 1]
         @test A(4.5) == u[:, 1]
+        test_cached_index(A)
     end
 
     @testset "Vector of Vectors case" for u in [
@@ -398,6 +420,7 @@ end
         @test A(3.5) == u[4]
         @test A(4.0) == u[4]
         @test A(4.5) == u[4]
+        test_cached_index(A)
 
         A = ConstantInterpolation(u, t; extrapolate = true) # dir=:left is default
         @test A(0.5) == u[1]
@@ -409,6 +432,7 @@ end
         @test A(3.5) == u[3]
         @test A(4.0) == u[4]
         @test A(4.5) == u[4]
+        test_cached_index(A)
     end
 
     @testset "Vector of Matrices case" for u in [
@@ -424,6 +448,7 @@ end
         @test A(3.5) == u[4]
         @test A(4.0) == u[4]
         @test A(4.5) == u[4]
+        test_cached_index(A)
 
         A = ConstantInterpolation(u, t; extrapolate = true) # dir=:left is default
         @test A(0.5) == u[1]
@@ -435,6 +460,7 @@ end
         @test A(3.5) == u[3]
         @test A(4.0) == u[4]
         @test A(4.5) == u[4]
+        test_cached_index(A)
     end
 
     # Test extrapolation
@@ -464,6 +490,7 @@ end
     @test A(-0.5) == P₁(-0.5)
     @test A(0.7) == P₂(0.7)
     @test A(2.0) == P₂(2.0)
+    test_cached_index(A)
 
     u_ = [0.0, 1.0, 3.0]' .* ones(4)
     u = [u_[:, i] for i in 1:size(u_, 2)]
@@ -496,6 +523,7 @@ end
     t = [-1.0, 0.0, 1.0]
 
     A = CubicSpline(u, t; extrapolate = true)
+    test_cached_index(A)
 
     # Solution
     P₁ = x -> 1 + 1.5x + 0.75 * x^2 + 0.25 * x^3 # for x ∈ [-1.0, 0.0]
@@ -552,6 +580,7 @@ end
         @test [A(25.0), A(80.0)] == [13.454197730061425, 10.305633616059845]
         @test [A(190.0), A(225.0)] == [14.07428439395079, 11.057784141519251]
         @test [A(t[1]), A(t[end])] == [u[1], u[end]]
+        test_cached_index(A)
 
         # Test extrapolation
         A = BSplineInterpolation(u, t, 2, :Uniform, :Uniform; extrapolate = true)
@@ -582,6 +611,7 @@ end
         @test [A(25.0), A(80.0)] ≈ [12.979802931218234, 10.914310609953178]
         @test [A(190.0), A(225.0)] ≈ [13.851245975109263, 12.963685868886575]
         @test [A(t[1]), A(t[end])] ≈ [u[1], u[end]]
+        test_cached_index(A)
 
         # Test extrapolation
         A = BSplineApprox(u, t, 2, 4, :Uniform, :Uniform; extrapolate = true)