Preparation False positive

Skin Test. Usually a battery of LAs is tested in the skin-prick test which is almost always negative. Then the intradermal test is performed with a 1 10 dilution of the substances. Undiluted LA preparations may commonly lead to false-positive reactions [30-32] in a rather high percentage of patients. 

Matrix effect is a phrase normally used to describe the effect of some portion of a sample matrix that causes erroneous assay results if care is not taken to avoid the problem or correct for it by some mechanism. The most common matrix effects are those that result in ion suppression and subsequent false negative results. Ion enhancement may lead to false positive results.126 127 Several reports about matrix effects include suggestions on what can cause them and how to avoid them.126-147 While various ways to detect matrix effects have been reported, Matuszewski et al.140 described a clear way to measure the matrix effect (ME) for an analyte, recovery (RE) from the extraction procedure, and overall process efficiency (PE) of a procedure. Their method is to prepare three sets of samples and assay them using the planned HPLC/MS/MS method. The first set is the neat solution standards diluted into the mobile phase before injection to obtain the A results. The second set is the analyte spiked into the blank plasma extract (after extraction) to obtain the B results. The third set is the analyte spiked into the blank plasma before the extraction step (C results) these samples are extracted and assayed along with the two other sets. The three data sets allow for the following calculations ... 

In the case that TER values below 5 kf2 are measured for surfactants or neutral organics, an assessment of dye (sulforhodamine B) penetration can be carried out to determine false-positive results [150], Preparation of the skin discs and the washing steps are known to be the critical steps. 

Diethylstilbestrol is particularly difficult to quantitate below 1.0 ppb in bovine tissues, especially in liver, which is among the last tissues to contain diethystilbestrol after cattle are withdrawn from receiving tire drug (101, 102). Interferences from tissue matrix constitute a major problem that might be due to nonspecific interference of lipids and fatty compounds (103, 104). In addition, problems with false-positive results often appear in urine analysis unless a chromatographic step such as a solid-phase extraction cleanup (105, 106) is introduced. Simple sample preparation procedures such as those based on solvent extraction and liquid-liquid partitioning do not usually give satisfactory results (107, 108). 

Meldal and co-workers26,27 first described the fluorescence-quenching approach to screen the OBOC combinatorial libraries for protease substrates. A fluorescence-quench combinatorial library prepared from the porous PEGA bead resin (see above) is first inspected under a fluorescent microscope and the fluorescent beads are removed and discarded. After all of the false-positive beads have been removed, the peptide beads are transferred into an Eppendorf tube and washed 5 x with water, followed by a 5 x wash with the appropriate protease buffer. Protease is then added... 

Establishes that laboratory contamination does Prepared with every preparation batch of up not cause false positive results to 20 field samples for all organic, inorganic,... 

Handling and preparation of authentic/proficiency test samples is preferably performed in a laboratory room dedicated to this purpose. The key advantage of a separate room is limitation of the possibilities of sample contamination. In previous tests, several participants have experienced cross-contamination, often with the consequence of false positive identifications and subsequent failure of the test. 

To reduce false-positive clones, mRNA should be prepared free from rRNA and genomic DNA as possible. We purify and concentrate mRNA by QuickPrep Micro mRNA purification Kit (Pharmacia) using glycogen as a carrier. Then, contaminated gemonic DNA is digested with RNase-free DNase I (Gibco-BRL). 

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