- Wide arrays of analytic methods are used to perform POCT, ranging from simple (e.g., pH paper for assessing amniotic fluid) to sophisticated (e.g., thromboelastogram for intraoperative coagulation assessment). Initially, POC tests consisted of traditional methods performed in the central laboratory that were simply transferred to POCT settings or put into smaller platforms to allow performance outside of the central laboratory. Subsequently, unique and innovative assays have been designed specifically for POCT (e.g., rapid streptococcal antigen test).
- Three tests comprise the majority of POCT in the US— urinalysis by dipstick, blood glucose and urine pregnancy. The following is an in-depth discussion on the dipstick urinalysis, the earliest and perhaps the most “basic” POC test. Similar discussions for other types of Point of Care testing will follow in upcoming editions of the tool kit.
B. Urinalysis Dipstick POC Testing: Past, Present and Future
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- In the 1940’s and early 1950’s several urine chemistry tests (e.g., albumin, occult blood and acetone) were developed utilizing dry reagent tablet technology from the pharmaceutical industry. Although these tests were primarily performed in the laboratory, the ability to place a reagent tablet in a tube, add urine, mix and visually read the result led to the ability to move urine testing back to the patient’s side. The next technological breakthrough was the discovery of the enzyme glucose oxidase which led to the first rapid and specific test for glucose which allowed for the detection and management of diabetes mellitus.
- Utilizing a combination of technologies from the newspaper industry liquid urine chemistry reagents were applied to paper and dried using an Egan tunnel (i.e., originally used to dry news paper ink). Reagent impregnated papers having different chemistries were slit into square pads which were laminated and adhered to a plastic backing. This was the advent of the first true POC device best known as the urine dipstick which was developed in 1957. A dipstick could contain one or more different chemistries but only required the user to dip the stick into a urine sample, remove and read the results visually at a set of given times.
- Over the next two decades additional chemistries were added to detect total protein, ketones, nitrite, specific gravity (SG) and leucocytes (WBC). Although visual detection of these strips was cost effective; visual acuity, reagent pad timing and color blindness made it clear that instrument read urine dipsticks would be required in the future. Advances in hardware e.g., integrated circuits (ICs), injection molding of plastics and light emitting diodes (LED), and software made it possible to develop a reflectometer which could read each urine strip pad individually at three to four wavelengths at the appropriate reaction time. These instruments eliminated the disadvantages of visually read urine dipsticks. It should be noted that eight of the twelve urine analytes have little practical diagnostic value; however, glucose, occult blood, leukocytes and protein can be useful screens for diabetes, bladder cancer, urinary tract infection and kidney disease. Due to the differences in urine concentration, all of the urine analyte measurements are semi-quantitative (i.e. analyte concentrations are given as ranges).
- Today several vendors offer a variety of urinalysis dipsticks having anywhere from one to twelve analyte test pads. Advances in high throughput manufacturing technologies during the last 15-20 years have allowed for the automated production of >100 million strips/year. Probably the most significant advancement in urinalysis dipstick testing in recent time was the development of a chemical method for the detection of urine creatinine. Urine creatinine has replaced specific gravity (SG) as a true measure of urine concentration which has allowed manufacturers to develop a new urine dipstick that measures the albumin to creatinine (A:C) ratio. Utilizing the A:C ratio allows for urine concentration correction which reduces both false positive and false negative microalbuminuria results, thereby improving the detection of early kidney disease.
- Although the basic urine dipstick technology has not changed over the years, vendors have improved products in three areas: analyte sensitivity, interference resistance and analyte chemistry cross talk. In order to improve urine screening for diabetes some vendors have lowered the sensitivity of the glucose pad. Most vendors continue to improve the sensitivity of their non-hemolysed and hemolysed occult blood in order to detect several disease states with bladder cancer at the top of the list. One of the biggest interferences in urine for some analytes is ascorbic acid (vitamin C). Some manufacturers have added compounds to the analyte pads to reduce or eliminate ascorbate interference. One vendor has also developed an ascorbic pad to indicate the presence of the interferent as a means to correct for it. Finally one vendor developed a sample spreading mesh that covers all analyte pads which ensures uniform dosing of the individual pads while reducing/preventing pad to pad cross talk.
- Advances in rapid prototyping (e.g., stereo lithography and soft tooling) have enabled manufacturers to quickly test product look and feel. Instrument displays were improved by using new technologies, such as liquid crystal display (LCD) and touch screens. Miniaturization of urine analyzers was driven by advances in the electronic industry. In the area of light detection, analyzers have used photodiodes, avalanche photodiodes, charge coupled detector (CCD) arrays and more recently CCD cameras. All of these technologies migrated into urinalysis from other consumer products (e.g., CCD arrays are used in barcode readers). In the area of optical illumination of urine dipsticks, the light emitting diode technology (e.g., chip on board LEDs and the white LED) has allowed manufacturers a low cost, more efficient and reliable alternative to incandescent light sources and filters. Optical grade plastics have significantly reduced the cost of lenses and waveguides compared to their glass counter parts. Some current urine analyzers have the ability to image the entire strip (e.g., 11 chemistries at once) rather than reading the individual pads.
- A couple of other improvements in the electronics industry like field programmable print circuit boards, increased memory, and computing power have led to major improvements in existing urine analyzers. Reprogramming mother boards, or a printed circuit board (PCB) allows for changes in next generation instruments (a.k.a., future proofing) and prevent costly PCB redesign. Increased memory capacity led to the ability to perform software upgrades using flash cards (i.e., technology developed for the digital cameras). Newer instruments have commercial computer operating systems onboard which allow increased computing power for complex algorithms like automatic strip identification. Symbology in the form of barcode readers has enabled urine analyzers to easily identify patients, operators, samples and product type. Finally a few of the recent instruments are highly connected via hardwired bi-directional serial ports and Ethernet connectors. In addition some systems are equipped with wireless connectivity in the form of Blue Tooth and WiFi. Technological advances during the last 15-20 years have allowed manufacturers to produce low cost urine analyzers that eliminate the subjective nature and associated errors with visually read urine dipsticks.
- Predicting the future in any area is difficult at best. The future changes will come in the areas of: improved reagent performance, miniaturization of urine analyzers, product format changes, reagentless detection and new analytes. For the most part this will be driven by the need to reduce sample handling while increasing the amount of clinically relevant information obtained from a given sample.
- In the area of improved strip performance we will see what is referred to as smart reagents and strips. Smart reagents will be more quantitative based on lot specific calibration of all analytes and will also have humidity and temperature checks that will not allow compromised strips to give test results (i.e., failsafe mechanism). Smart strips will utilize “watermark” technologies, such as, 2-D barcodes, IR dyes, hologram and/or powder radio frequency identification device RFID for individual strips that will include lot number, lot calibration, expiration date and strip identification. Finally smart strips will use clinical diagnostic algorithms to identify patients with urinary tract infections, early stage kidney disease, and diabetics with ketoacidosis.
- Future urine analyzers will be small portable instruments that will be designed using a technology referred to as “naked or nude electronics”. These are based on techniques like SHE’D which was used on the iPod and iPhone. SHE”D stands for shrink, hide, eliminate and define. It is possible in the future that we will see an iPhone-like device capable of reading urine strips via its digital camera and being fully connected so it can send the data to care givers wirelessly.
- In order to minimize urine sample handling in the future, at least three possible approaches are to miniaturize the urine dipstick thereby reducing urine volume requirement, placing the reagent detection technology into a urine collection cup (a.k.a., smart cup) and finally utilizing reagentless technology in a “smart toilet”. There are several potential means to miniaturize existing urine strips (e.g., make them skinnier or shorter) but the use of new dispensing technologies and substrates could make it possible to develop a strip having one pad (current size) with all 10-15 analytes that could be read on small portable instrument. A smart cup would have all of the analyte detection technology in the cup so that a patient would urinate in the cup and the cup would be instrument read (e.g., optically or electrochemically), thereby minimizing sample handling. The ultimate sample handling solution would be the smart toilet which is in development. The smart toilet will measure at least three to four analytes in diluted urine collected by the toilet via near- or mid-infrared spectroscopy. Reagentless technology utilizing near infrared NIR or mid infrared MIR spectroscopy could be utilized to detect several urine analytes and therefore completely change current technologies.
- Finally we will see several new urine analytes that will require alternative technologies (non-dipstick) to perform. Today some other urine analytes are detected using lateral flow immunoassay technology, such as, human chorionic gonadotropin (hCG), Drugs of Abuse Urine (DAUs), microalbuminuria (A:C ratio) and human immunodeficiency virus (HIV). Some urine testing is being directed toward alternative sample types like saliva and oral fluid (e.g., Drugs of Abuse and HIV). It is likely we will see new analytes requiring lateral flow immunoassay or molecular technologies (e.g., lab on a chip). New analytes will be in the area of preeclampsia [e.g., soluble fms-like tyrosine kinase-1 (sFlt-1), placental growth factor (PLGF) and endoglin), prostate cancer (e.g., PCA-3) and pancreatitis.
- The above illustrates that this “simple” test is really a very sophisticated one with potentially dazzling upgrades in the future. The similar discussions on glucose meters and urine pregnancy testing will demonstrate the same principles. Hopefully, these examples will give the reader an insight into how technically advanced all these “easy” devices can be and how much exciting technology is invested in them. Future editions will summarize existing and future technologies for analytes by medical disciplines. The tool kit will try to demonstrate the pros and cons of various systems. We also hope that the discussions will give users new respect for POCT and allow them to compare and evaluate test systems in their own environments. We hope that users will appreciate the need to treat POC test systems with the same attention to detail routinely associated with traditional central laboratory testing.