Assuming your string always as mA at the end, the first thing to do is get rid of it. There are a load of different ways, and a Regex is probably the most flexible, but...if the string always does end "mA" then you can cheat.
string input = "0.000038mA";
double d = Convert.ToDouble(input.Substring(0, input.Length - 2));
int value = (int)(d * 1000000);
You could skip the double conversion:
string input = "0.000038mA";
int value = Convert.ToInt32(input.Substring(2, input.Length - 4));
But...
[edit]
Sorry, I forgot you love VB so much...
Dim input As String = "0.000038mA"
Dim d As Double = Convert.ToDouble(input.Substring(0, input.Length - 2))
Dim value As Integer = CInt(Math.Truncate(d * 1000000))
And
Dim input As String = "0.000038mA"
Dim value As Integer = Convert.ToInt32(input.Substring(2, input.Length - 4))
[/edit]