Proportional versus projected glucose tolerance values: a quality control program

The objective of this study was to improve the reproducibility of the oral glucose tolerance test (OGTT).

Ten patients were used to demonstrate the difference between proportional and projected values in an OGTT. A procedure that plots data points at their actual time of collection, producing an accurate graph, that can predict a proportional glucose value at the intersection of the curve with the two-hour specified collection time, was developed. The efficacy of this procedure was demonstrated by clinical trials on a second group of ten patients.

Hospital outpatient clinic for the first group of ten patients since heparin lock was not required; Columbia Medical Center, Ormond Beach, FL for the second group since heparin lock was required.

All subjects in the first group were referred to the clinic by their family physician for an OGTT. Subjects in the second group were selected to meet the following criteria: no overt symptoms of diabetes, but with at least two high risk factors for Type 2 diabetes.

A glucose load was given to each patient after a fasting sample blood sugar was analyzed.

The recommended OGTT procedure for obtaining accurate predicted proportional two hour plasma glucose (2hPG) values requires a three hour OGTT with 1/2 hourly mid-test samples. A sample collection form was used to record all pertinent aspects of the test, time of first swallow, amount ingested in five minutes and the exact times of sample collections. A digital clock, located in the phlebotomist's view, was used to accurately establish all time intervals, including zero time.

The study demonstrated that inspection of the OGTT curve and calculated values provide additional useful information. It also identified five impaired glucose tolerance (IGT) subjects, one gestational diabetes mellitus (GDM) patient, thirteen normal subjects, and one flat curve.

Accurate time intervals are not necessary since 2hPG can be accurately expressed as a predicted proportional value if the true intervals are recorded and used to calculate the glucose level at the desired time interval(s).