|Image by Wolfgang Ihloff in Wikipedia|
SO... when a skin prick test is performed, a doctor takes a needle and dips the tip of it into this "essence of cat" and pricks the skin of a patient depositing a small amount under the skin.
Assuming a patient is ONLY allergic to cat Epitope A (which the vast majority of patients are), both the skin prick and blood testing will give the same exact positive result.
HOWEVER, if a patient is ONLY allergic to Epitopes b and F but not A, than the skin prick test will still come back positive, BUT the blood test will come back normal! Remember, the blood test can only detect Epitope A, but not b and F.
So that's why a skin prick and blood test may disagree on whether a patient may be allergic to something or not.
And before you ask... skin prick testing typically checks more epitopes for a given substance like cat than blood testing.
Given your newfound knowledge now, can you figure out why BOTH skin prick AND blood testing may come back normal even if a patient may truly be allergic?
It rare situations, a patient may theoretically be allergic to other epitopes not present in the test. For example, what if a patient is allergic to cat Epitope R, x, and W? If that's the case, a skin prick test for cat Epitopes A, b, and F and blood test for cat Epitope A will both come back normal.
If this last situation applies to you, there may not be any good way to get tested for more unusual epitopes other than to go to a tertiary care allergy center. Hopefully with time, biotech companies will develop even better tests that incorporate even more if not all epitopes that may trigger allergic reactions in humans.