The cellular components of body fluids are routinely analyzed to identify disease and treatment approaches. While significant focus has been placed on developing cell analysis technologies, tools to automate the preparation of cellular specimens have been more limited, especially for body fluids beyond blood. Preparation steps include separating, concentrating, and exposing cells to reagents. Sample preparation continues to be routinely performed off-chip by technicians, preventing cell-based point-of-care diagnostics, increasing the cost of tests, and reducing the consistency of the final analysis following multiple manually-performed steps. Here, we review the assortment of biofluids for which suspended cells are analyzed, along with their characteristics and diagnostic value. We present an overview of the conventional sample preparation processes for cytological diagnosis. We finally discuss the challenges and opportunities in developing microfluidic devices for the purpose of automating or miniaturizing these processes, with particular emphases on preparing large or small volume samples, working with samples of high cellularity, automating multi-step processes, and obtaining high purity subpopulations of cells. We hope to convey the importance of and help identify new research directions addressing the vast biological and clinical applications in preparing and analyzing the array of available biological fluids. Successfully addressing the challenges described in this review can lead to inexpensive systems to improve diagnostic accuracy while simultaneously reducing overall systemic healthcare costs.

Antimicrobials remain an integral part of the treatment of patients with an infection. New microfluidic technologies are poised to help clinicians prescribe the right antimicrobials, sooner, reducing long hospital stays and improving outcomes. Given that current microbiologic diagnostic testing methods require a significant turnaround time (days), clinicians, in general, initially empirically determine a suitable therapy. After review of laboratory data, including information regarding the susceptibility of the microbial pathogen to specific anti-infectives, a clinician will then make alterations in therapy as appropriate, to direct therapy toward the pathogen involved in the illness. Important steps needed to quickly ascertain this information include the timely isolation of the microorganism, followed by direct antibiotic susceptibility tests (ASTs) or determination of the presence within the microorganism of any resistance genes or proteins that will impair the activity of a potential therapy. Recent microfluidic technologies highlighted here that can intrinsically interface at the scale of the organisms are starting to address these challenges by improving the speed and accuracy of tests aimed at helping physicians to give the right antimicrobials sooner.

Interleukin-6 blockade (IL-6) has become a focus of therapeutic investigation for the coronavirus-2019 (COVID-19). We report a case of a 34 year-old with COVID-19 pneumonia receiving an IL-6 receptor antagonist (IL-6Ra) who developed spontaneous colonic perforation. This perforation occurred despite a benign abdominal exam and in the absence of other known risk factors associated with colonic perforation. Examination of the colon by electron microscopy revealed numerous intact SARS-CoV-2 virions abutting the microvilli of the colonic mucosa. Multiplex immunofluorescent staining revealed the presence of the SARS-CoV-2 spike protein on the brush borders of colonic enterocytes which expressed angiotensin-converting enzyme 2. However, no viral particles were observed within the enterocytes to suggest direct viral injury as the cause of colonic perforation. These data and absence of known risk factors for spontaneous colonic perforation implicate IL-6Ra therapy as the potential mediator of colonic injury in this case. Furthermore, this report provides the first in situ visual evidence of the virus in the colon of a patient presenting with colonic perforation adding to growing evidence that intact infectious virus can be present in the stool.

In this issue we highlight three recent papers that demonstrate new strategies to extend the capabilities of paper microfluidics. Paper (a mesh of porous fibers) has a long history as a substrate to perform biomolecular assays. Traditional lateral flow immunoassays (LFAs) are widely used for rapid diagnostic tests, and perform well when a yes or no answer is required and the analyte of interest is at relatively high concentrations. High concentrations are required because usually only a small volume of analyte-containing fluid flows past the detection region, leading to a limited signal. Further, the small pores within paper matrices prevent the use of paper to control the flow of larger particles and cells, limiting the use of paper microfluidics for cell-based diagnostics. The work we highlight addresses these important unmet challenges in paper microfluidics: enriching low concentration analytes to a higher concentration in a smaller volume that can be processed effectively, and using paper to pump flows in larger channels amenable to cells. Applying these new approaches may allow diagnosis of disease states currently technically unachievable using current LFA systems, while maintaining many of the "un-instrumented" advantages of an assay on self-wicking paper.