Current screen used: plasma glucose 1 hour after a 50 gram oral glucose load
Parameters for new index based on testing 1 hour after a 50 gram oral glucose load:
(1) plasma glucose in mmol/L
(2) total protein in g/L
(3) fructosamine in µmol/L
index =
= (fructosamine) * (glucose) / (total protein)
where:
• The original formula had a value of 100 in the denominator. However, when values are used in the stated units having this in the denominator does not give the correct index values.
NOTE: The fructosamine assay used is different from the "fructosamine" assay used in the USA.
• The assay referenced in the US measures glucose linked to protein. According to Tietz's Clinical Guide to Laboratory Tests and Jacobs & Demott's Laboratory Test Handbook values for a normal person can go up to 2.7 mmol/L or 2,700 µmol/L.
• The assay used in this paper appears to detect a modified fructose molecule. According to Young and Huth it is reported in mg/L and the conversion factor to µmol/L is 5.581. This gives a molecular weight of around 179.1. The molecular weight for D-fructose is 180.16. The reference range in Young and Huth is given as 202-282 µmol/L.
Interpretation:
• The decision value for the index was 27.2.
• An oral glucose tolerance test (OGTT) is performed on women with a positive index.
|
Index
|
Interpretation
|
|
< 27.2
|
negative; sensitivity 98%
|
|
>= 27.2
|
positive; specificity 88.8%
|
|
> 30.5
|
gestational diabetes mellitus with specificity of 96%
|
Performance (Table 1, page 437):
• The sensitivity is comparable to the oral glucose screening test, but the specificity and positive predictive value are superior.
Limitations:
• The index would be affected by variation in the fructosamine assay. The assay used in the study was a tetrazolium blue colorimetric assay read at 552 nm.
