One doesn't usually think of complete blood counts (CBCs) or coags to require the patient to fast. Researchers in Latin America, however, found evidence to the contrary.

Twenty healthy volunteers participated in their study on the effect breakfast has on routine hematology and coagulation laboratory testing. A breakfast containing a standardized amount of carbohydrates, proteins, and lipids was consumed 1, 2, and 4 hours before blood sampling for routine hematology and coagulation testing. Statistically significant differences were found two hours after breakfast for red blood counts, hemoglobin, hematocrit and mean corpuscular volume.  Four hours after ingesting the breakfast meal, statistically significant differences were found for red blood cells, hemoglobin, hematocrit, mean corpuscular volume, white blood cells, neutrophils, lymphocytes, monocytes, mean platelet volume, and activated partial thromboplastin time.

The authors concluded fasting needs to be carefully considered prior to drawing CBCs and coagulation tests and recommend it for coagulation testing. 

Read the full study.

[Editor's note: implementing the results of any singular study with a small sample size may not be warranted without further and more comprehensive evidence from multiple studies reach the same conclusion.]