This effect can cause false negatives in pregnancy tests when the hCG concentration is above 500 international units per milliliter of urine. For 10 points each:
[10h] Name this unexpected decrease in signal from a sandwich assay when the target concentration exceeds antibody concentration. This infuriating effect is named for the shape of the resulting dose-response curve.
ANSWER: hook effect [accept prozone effect or prozone phenomenon; or postzone phenomenon]
[10m] The hook effect skews interpretation of these simple immunoassays in which sample flows by capillary action past antibodies immobilized on nitrocellulose onto a wicking pad. Pregnancy tests and rapid antigen tests are this type of assay.
ANSWER: lateral flow test [or lateral flow assay; or LFA or LFD or LFT; prompt on dipstick]
[10e] Signal in a lateral flow test is often generated by red, antibody-conjugated nanoparticles of this element. A colloid of this noble metal can label antigens for electron microscopy.
ANSWER: gold [or Au; or gold nanoparticles or colloidal gold]
<VD, Chemistry>