What is the right way to use Nullable in F#?

Currently I'm using this, but it seems awefully messy.

let test (left : Nullable<int>) = if left.HasValue then left.Value else 0

Console.WriteLine(test (new System.Nullable<int>()))
Console.WriteLine(test (new Nullable<int>(100)))

let x = 100
Console.WriteLine(test (new Nullable<int>(x)))
f# null
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