Sub Con()
 Dim X As Long
 Dim rst As Recordset

 X = 877717 'в реале Х - вычисляемая переменная 
Set rst = CurrentDb.OpenRecordset("SELECT * FROM tPrescripts WHERE idPrescript = 877717")
 'Set rst = CurrentDb.OpenRecordset("SELECT * FROM tPrescripts WHERE idPrescript = X") '- НЕ РАБОТАЕТ
MsgBox rst.RecordCount
End Sub