Dried blood spot samples provide a simple and relatively inexpensive way to collect and store blood by using specially designed filter paper, and are routinely created for infants born in the United States. Extra dried blood spot samples are often stored by state departments of health for several years, and offer a potential resource for epidemiological research. This study measured two analytes of interest, folate and hemoglobin, in dried blood spot samples from healthy Minnesota newborn subjects that were stored for 9 months at four different conditions: -80 degrees C, 4 degrees C, 25 degrees C, and a humid condition. We found that folate levels decreased significantly with warmer storage temperatures and humidity, while hemoglobin levels decreased with increased storage temperature but not humidity.
The hemoglobin-folate value, used to standardize the amount of blood in the sample for measuring folate, did not decrease the standard error for our samples. Thus, for studies measuring folate using a constant size of saturated dried blood spot filter paper, measuring hemoglobin may not offer an advantage. Despite the decrease in measured values, rank order of hemoglobin, folate, and hemoglobinfolate measurements was generally preserved between storage conditions, as analyzed by general estimating equations. This indicates that dried blood spots stored under unfavorable conditions retain the ability to distinguish between differing levels of folate and hemoglobin and so may still be useful for epidemiological studies.