Complete blood count analyses many factor and hence the interpretation is complex. Along with factors analysed in the test, a phycisian will always ask you to do other tests also taking into account patient’s signs and symptoms and medical history to come to conclusion.
All factors analyzed together are used to diagnose the condition or prevent the onset of any critical condition. Complete blood count is particularly useful in health monitoring of elderly patients or patients suffering from chronic illness as it detects even slightest of the variations making it a great preventive checkup test.

No fasting is required. However, other test done along with this may require it, hence check with your healthcare provider for the same.

Complete blood count reference range:

Hemoglobin Men: 13 to 17 (gm/dL) for Women: 12 to 15 gm/dL
Red Blood Cells (RBCs) Men: 4.5 to 5.5 million cells/mcL;
Women: 3.8 to 4.8 million cells/mcL
White Blood Cells (WBCs) 4,000 to 10,000 (cells/mcL)
Platelets 1,50,000 to 4,10,000 platelets/mcL
Hematocrit Men: 40% to 50%;
Women: 36% to 46%
Mean corpuscular volume (MCV) 83 to 101 fL
Mean Platelet Volume (MPV) 6.8 - 10.9 fL
Mean Cell Haemoglobin 27 - 32 pg
RBC Distribution Width 11.6 - 14%

Your healthcare provider may ask you to get CBC as part of regular health checkup if you are suffering from following conditions:
1. Prolonged bleeding for minor injuries
2. Taking Chemotherapy for cancer
3. Leukemia
4. Excessive bleeding
5. Nose bleeding
6. Red or purple spots on skin
7. Unexplained bruising
8. Anemia
9. Viral infections

CBC is very useful in patients with bleeding disorders, patients taking medications which impact blood cells and for patients having cancer, bone marrow disease, bleeding disorders or autoimmune disorders. Parameters such as MPV, platelet crit are crucial for monitoring bleeding disorders and bone marrow conditions. Differential counts of WBCs help evaluate infections and severity of the same.