Nephelometric immunoassay for therapeutic drug level monitoring

Nephelometric immunoassay for the determination of drug levels in blood is based on the inhibition of immunoprecipitation by a hapten (drug). It represents a homogeneous method that does not require any separation steps nor radioisotopes. Precipitation in an aqueous solution can be quantitated by nephelometry (scattered light measurement) or turbidmetry (traversed light measurement). Advantages over other drug assay methods include its simplicity, speed and low cost. Only two reagents are added, and the subsequent reaction can be monitored optically with the potential for full automation. The reaction is usually completed in less than 15 minutes. The two reagents, anti-drug antibody and polyhaptenic antigen, can be easily prepared and are highly stable. Therefore, precipitation inhibition immunoassays and in particular nephelometric immunoassays are being commercially developed for routine therapeutic monitoring of drugs such as anticonvulsant drugs, aminoglycoside antibiotics and theophylline. The specificity is high, though depending on the cross-reactivity of the anti-drug antibody as is the case with other immunoassays. The sensitivity depends on a variety of factors such as antibody-hapten affinity, detection mode of the precipitation, and intrinsic turbidity of the test sample. But the sensitivity is sufficiently high for serum drug concentration greater than 1 µg/ml when less than 10 µl of serum are used. Variations of this assay technique include rate analysis for precipitate formation instead of endpoint analysis. Agglutination-, or particle aggregation-inhibition immunoassay is also a useful and more sensitive method. Finally, use of monoclonal antibodies can serve to enhance the specificity of nephelometric immunoassay of drugs.