A blood test can confirm an infection in cases where a shingles diagnosis is difficult due to lack of rashes and blisters, but the test cannot conclusively diagnose the infection as shingles, says HealthinAging.org. With blisters, health care providers scrape them to get samples to test.
In many cases, shingles is diagnosed via skin examinations, medical histories and an examination of symptoms, according to HealthinAging.org. Sometimes the shingles infection is so painful and intense that a patient feels as though he is having a heart attack before any rashes present themselves. Treatment that begins within 24 hours before the appearance of any rash is usually most effective, although treatment within 72 hours tends to help greatly as well.