Sample pre-treatment determines the clinical usefulness of acid-labile subunit immunoassays in the diagnosis of growth hormone deficiency and acromegaly.
OBJECTIVE
The usefulness of measuring the GH-dependent acid-labile subunit (ALS) in the management of GH deficiency (GHD) and acromegaly remains in question and is investigated in this study, comparing several different immunoassays for ALS.


METHODS
We compared the diagnostic accuracy of a commercially available polyclonal Ab-based ELISA with SDS pre-treatment (SDS-ELISA) with a monoclonal Ab-based immunofluorometric assay, using two unfolding methods (urea (UREA) and Glycine-HCl (Gly)). The corresponding molecular weight (MW) of ALS and IGFBP-3 immunoreactivity was determined. The clinical usefulness of each assay was examined in adult GH disorders.


RESULTS
ALS was lower in GHD and higher in acromegaly using all assays. In GHD, UREA had higher sensitivity and specificity than SDS-ELISA (59 and 69% versus 41 and 51% respectively). In acromegaly, sensitivity and specificity was 94 and 87% for UREA, 81 and 36% for Gly, and 44 and 44% for SDS-ELISA. After UREA, immunoreactivity for ALS and IGFBP-3 eluted at their predicted free MW using size-exclusion chromatography, whereas ALS immunoreactivity in SDS (300-600 kDa) and Gly (250-500 kDa) was at a high apparent MW consistent with aggregation.


CONCLUSIONS
The diagnostic accuracy of ALS varies with assay choice and pre-treatment modality. UREA, which results in migration of ALS at the expected MW on a sizing column, has the highest specificity and sensitivity. Thus, if measured in an assay in which ALS is unfolded without aggregation, ALS is a clinically highly useful parameter for the assessment of GH.
Morrison
Katherine M
KM
Department of Pediatrics and Population Health Research Institute, McMaster University Medical Centre, Room 3G52, 1200 Main St. W. Hamilton, Ontario, Canada L8N 3Z5. morriso@mcmaster.ca
Bidlingmaier
Martin
M
Stadler
Simone
S
Wu
Zida
Z
Skriver
Lars
L
Strasburger
Christian J
CJ
D000172:Acromegaly / Q000097:blood / Q000175:diagnosis*; D000328:Adult; D002352:Carrier Proteins / Q000097:blood* / Q000737:chemistry; D002850:Chromatography, Gel; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods; D005615:Freezing; D006023:Glycoproteins / Q000097:blood* / Q000737:chemistry; D019382:Human Growth Hormone / Q000097:blood / Q000172:deficiency*; D006801:Humans; D007118:Immunoassay / Q000379:methods; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000097:blood / Q000737:chemistry; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
D002352:Carrier Proteins; D006023:Glycoproteins; D018972:Insulin-Like Growth Factor Binding Protein 3; C075944:insulin-like growth factor binding protein, acid labile subunit; D019382:Human Growth Hormone