Performance indicators and validity of serum fructosamine assay as a diagnostic test in a screening program for diabetes mellitus

OBJECTIVE: To evaluate the performance indicators and validity of fructosamine assay as a diagnostic tool in screening for Diabetes mellitus (DM).

METHODS: Fasting plasma glucose (FPG) and serum fructosamine (FA) were compared in 1015 subjects aged >= 25 years from different urban and rural areas in Mosul city, Northern Iraq. The subjects were classified into 5 groups: Group 1: Subjects with FPG < 6.1 mmol/L (n=883), Group 2: Subjects with impaired FPG 6.1-6.9 mmol/L (n=29), Group 3: New diabetics diagnosed solely by new 1997 American Diabetes Association (ADA) criteria with FPG 7.0-7.7 mmol/L (n=20), Group 4: New diabetics diagnosed according to old 1980-1985 World Health Organization (WHO) criteria with FPG >= 7.8 mmol/L (n=23), and Group 5: Known diabetics (n=60). Subjects in groups 2 and 3 underwent a standard 75 gm oral glucose tolerance test (OGTT) as recommended by the WHO. Reclassification of subjects into 3 groups according to FPG or 2hPG, or both was carried out for all subjects. Group A (non-diabetics): Subjects with FPG < 6.1 mmol/L or 2hPG < 7.8 mmol/L, or both (n=910). Group B (Diabetics): Subjects with FPG >= 7.8 mmol/L or 2hPG >= 11.1 mmol/L, or both (n=92) including 60 known diabetics in group 5 and 23 new diabetics in group 4 in addition to 2 subjects in group 2 and 7 subjects in group 3. Group C (impaired glucose tolerance, IGT): Subjects with 2hPG between 7.8-11.1 mmol/L (n=13).

RESULTS: Having all subjects had their serum FA being measured; the Receiver Operator Characteristic (ROC) curve was constructed on the data to determine the trade off between sensitivity and specificity of the FA test in the diagnosis of DM. This construction decided that serum FA value of 2.65 mmol/L would be the cutoff point, or the positivity criterion in the calculation of the validity parameters of FA test. Of 910 non-diabetics, 886 subjects had measured FA values within the 95th percentile, while 24 had FA higher than the cutoff point. Consequently, FA in non-diabetics yielded 886 (true negatives) and 24 (false positives). Of the 92 diabetics, 30 subjects had normal FA values, while 62 diabetics showed FA higher than the cutoff point. Consequently, FA in diabetics yielded 30 (false negatives) and 62 (true positives). Accordingly, the sensitivity, specificity, positive predictive value, negative predictive value, accuracy rate, positive likelihood ratio and negative likelihood ratio were 67.3%, 97.3%, 72.3%, 96.7%, 94.6%, 26 and 2.99. A highly significant correlation was observed between FPG and measured FA in non-diabetics (r=0.85, p<0.0001) and diabetics (r=0.92, p<0.0001). No significant correlation was observed between serum FA and albumin in non-diabetics (r= 0.14, p>0.05) and diabetics (r=0.08, p>0.05).

CONCLUSION: Fructosamine test shows a moderate sensitivity with a high specificity as a diagnostic test for diabetes mellitus. The considerable overlap between diabetics and non-diabetics limit its usefulness. It is recommended that fructosamine test is not a suitable screening test for the disease. Measurement of plasma glucose (fasting or post-OGTT) remains the corner stone as a diagnostic test.