Many types of ligand-receptor assays have been used to detect the presence of various substances in body fluids, such as urine, saliva, or blood. Many tests are designed to make a quantitative determination, but in many circumstances all that is required is a qualitative positive/negative indication. Examples of such qualitative assays include blood typing, pregnancy testing, and many types of urinalysis. For these tests, visually observable indicia, such as the presence of agglutination or a color change, are preferred.
The positive/negative assays must be very sensitive because of the often small concentration of the ligand of interest in the test fluid. False positives can be troublesome, particularly with agglutination and other rapid detection methods such as dipstick and color change tests. Because of these problems, sandwich assays and other sensitive detection methods which use metal sols or other types of colored particles have been developed.
U.S. Pat. No. 6,485,982, which is incorporated herein by reference in its entirety, describes a diagnostic test cell formed of an elongate outer casing which houses an interior permeable material (such as glass fiber) capable of transporting an aqueous solution by capillary action, wicking, or simple wetting. The casing defines a sample inlet, and interior regions, which are designated as a test volume and a reservoir volume. The reservoir volume is disposed in a section of the test cell spaced apart from the inlet and is filled with sorbent material. The reservoir acts to receive liquid transported along a flow path defined by the permeable material and extending from the inlet and through the test volume. In the test volume is a test site comprising a first protein having a binding site specific to a first epitope of the ligand immobilized in fluid communication with the flow path (e.g., bound to the permeable material or to latex particles entrapped in or bonded to the permeable material). A window, such as a hole or transparent section of the casing, permits observations of the test site through the casing wall. The method of use of the test cell requires the use of a conjugate comprising a second protein bound to colored particles, such as a metal sol or colloid, preferably gold. The conjugate can take two distinct forms, depending on whether the assay is designed to exploit the “sandwich” or “competitive” technique.
U.S. Pat. No. 7,045,342, which is incorporated herein by reference in its entirety, describes a diagnostic device including a biphasic chromatographic medium. The biphasic substrate is formed of a release medium joined to a capture medium located downstream of the release medium. The release and capture media preferably comprise two different materials, or phases, having different specific characteristics. The two phases are joined together to form a single liquid path such that a solvent front can travel unimpeded from the proximal (upstream) end of the release medium to the distal (downstream) end of the capture medium.
Although diagnostic devices, such as those described above, show improvements over the art, there still remains a need for test devices providing greater accuracy and detection of even lower analyte levels in the sample fluid. For example, in the field of pregnancy testing, accurate and rapid detection of low levels of hCG is desired to allow consumers to confirm pregnancy early after conception has occurred.