Nothing stands out as obvious why that code wouldn't work, but there's a lot of context missing and I'm assuming that the values such as 'sickHigh' are defined elsewhere. Do you have the project on Github or another location?
Also, that 'else return' statement will never trigger... but if you are really worried about performance, you might want to just look at mapping the the range of values to an array and then just bound checking as needed. Returning the value from the array is always going to be faster than any sort of conditional logic. However, typically the rule of thumb is to avoid premature optimization of the code in general - readable code in models is always preferable when it comes to peer review.