Hematocrit-Independent Sampling Enables White Blood Cell Counts from Patterned Dried Blood Spot Cards

The accurate and efficient measurement of white blood cell (WBC) counts is vital for monitoring general patient health and can aid in the diagnosis of a range of possible infections or diseases. Even with their importance universally acknowledged, access to WBC counts is largely limited to those with access to phlebotomists and centralized clinical laboratories, which house the instruments that perform the tests. As a result, large populations of people (e.g., those that are home-bound or live in remote locations) lack facile access to testing. Dried blood spot (DBS) cards are often used to bridge these gaps in access to testing by offering the ability to collect blood at home for ambient shipping to laboratories. However, it is well understood that these cards, which are prepared from cellulose cardstocks without further modification, suffer from variabilities in accuracy and precision due to uncontrolled sample spreading and hematocrit effects, which have hindered their use to determine WBC counts. In this paper, we present a method to obtain an accurate WBC count using a patterned dried blood spot (pDBS) card, which comprises collection zones that meter volumes of dried blood. Using an input volume of 75 μL of whole blood, we demonstrate that, unlike the gold standard DBS card (Whatman 903), our pDBS design allows for the collection of replicate zones containing a reproducible, average volume of dried blood (12.1 μL, 7.8% CV) over the range of hematocrits from 25 to 55%. We then used qPCR to quantify the 18S rRNA gene to determine WBC counts from the volumes of blood that are metered in pDBS zones. We observe that WBC counts generated from our method are comparable to those measured with a HemoCue point-of-care WBC analyzer. Our approach to using pDBS cards as a blood collection device has the potential to support at-home sampling and other patient populations that need WBC counts but lack access to clinical facilities.