Reactivity of urinary albumin (microalbumin) assays with fragmented or modified albumin.
BACKGROUND
Controversy exists regarding occurrence and measurement of structural variants of albumin in urine. In this study, we examined cross-reactivity of in vitro modified albumins in assays for urine albumin (microalbumin).


METHODS
We analyzed albumin modified by reagents, trypsin, or physical treatments or differing in primary sequence (animal albumins) with an immunoturbidimetric assay (Beckman LX20) using goat antiserum and a competitive immunoassay (Siemens Immulite) using a monoclonal antibody. We assessed occurrence of albumin fragments in urine by use of Western blotting of 24 specimens.


RESULTS
Chemical modification, modest sequence substitution (gorilla albumin), or cleavage of albumin by cyanogen bromide (CNBr) had little effect on reactivity in the LX20 assay. Albumin extensively cleaved with trypsin retained partial reactivity. The Immulite assay generally was affected more severely by albumin modifications and sequence changes. Western blots of fresh urine specimens or specimens stored at -80 degrees C showed little albumin fragmentation, but some specimens stored for 3 years at -20 degrees C had extensively fragmented albumin that was detected by the LX20 but not the Immulite assay.


CONCLUSIONS
Nearly equivalent reactivity of intact albumin and CNBr fragments in the immunoturbidimetric assay indicates reactivity of antibodies with multiple epitopes throughout albumin. Therefore, it is difficult to abolish reactivity of albumin in this type of urine albumin assay. Differential sensitivity of 2 assays to albumin modification identifies a potential source of assay nonequivalence in measuring urinary albumin, particularly for specimens stored at -20 degrees C.
Sviridov
Denis
D
Departments of Laboratory Medicine and Critical Care Medicine, Warren Magnuson Clinical Center, National Institutes of Health, Bethesda, MD 20892, USA.
Drake
Steven K
SK
Hortin
Glen L
GL
D000419:Albuminuria / Q000175:diagnosis* / Q000652:urine; D000818:Animals; D003429:Cross Reactions; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D007118:Immunoassay; D009391:Nephelometry and Turbidimetry; D010446:Peptide Fragments / Q000652:urine; D012680:Sensitivity and Specificity; D012709:Serum Albumin / Q000032:analysis*; D013045:Species Specificity
D010446:Peptide Fragments; D012709:Serum Albumin