Umm, not that I use these, but just a quick glance at the online resources available to us, which might be available for you:
Differential Diagnosis--Laboratory Tests
MICROMEDEX: Select "Lab" from menu
A Manual of Laboratory and Diagnostic Tests: via Books@Ovid
Interpretation of diagnostic tests: via Books@Ovid
AccessMedicine: select "Dx Tests" from menu
MGH Normal Reference Laboratory Values
The two bolded I looked at.
The first one basically seems like a comprehensive resource. Basically it'll give you the nitty gritty about every lab test. You could buy a hardcover if you so desired. It's on amazon.
The access medicine resource seemed more like a quick hit resource.