The science behind the Cairn Gastric Emptying Breath Test
After an overnight fast and providing duplicate pre-meal breath samples, the patient consumes the 13C-Spirulina Gastric Emptying Breath Test (GEBT) meal. This meal consists of 27 grams of re-hydrated, precisely formulated, pasteurized scrambled egg mix containing a dose of 43 mg of 13C (provided by approximately 100 mg of 13C-Spirulina), 6 saltine crackers and 6 ounces (180 mL) of potable water. The caloric value of the meal is approximately 230 kCal.
As the egg meal containing the 13C-Spirulina is triturated by the stomach to a particle size of 1 – 2 mm, it passes through the pylorus into the intestine. In the upper small intestine, the labeled products of 13C-Spirulina digestion (proteins, carbohydrates and fats) are absorbed and subsequently metabolized, giving rise to 13C-labeled carbon dioxide (13CO2) expired in the breath. Breath samples, collected periodically in capped glass tubes before and after test meal administration, are returned to a central laboratory for analysis by gas isotope ratio mass spectrometry (GIRMS) to determine the ratio of 13CO2 to 12CO2 in each sample.
By measuring the change in this ratio over time as compared to the pre-meal value, the rate of 13CO2 excretion can be calculated and the individual’s gastric emptying rate determined. The rate of gastric emptying is proportional to the rate of 13CO2 excretion at any measurement time “t.”
Cairn GEBT test results are reported using the metric “kPCD.” At any measurement time t, kPCD(t) = 1000 X [Percent carbon-13 dose (PCD) in test meal excreted (as 13CO2) per minute]. A larger kPCD value means a faster 13CO2 excretion rate which is proportional to a faster rate of gastric emptying.
Once the clinical laboratory has assayed each of the patient’s breath samples, test results are presented as 13CO2 excretion curves, depicting excretion rate (kPCD per minute) over time. Increasing rates of 13CO2 excretion (kPCD min-1) reflect increasing rates of gastric emptying. The location and shape of the patient’s curve relative to reference range cut-off points (COP’s) provide data for identifying gastroparesis.
Subjects with NORMAL rates of gastric emptying (Exhibit I) typically display kPCD values that exceed time-specific cut-off point(s) (COPs), reach a maximum kPCD value between 120 and 180 minutes, and then decline.
Exhibit I: Subject with NORMAL Rate of Gastric Emptying
In contrast, kPCD values of DELAYED patients (Exhibit II) are lower and typically rise continuously throughout the four-hour post-meal evaluation period. As a result, their highest kPCD value will occur at four hours.
Exhibit II: Subject with VERY DELAYED Rate of Gastric Emptying
Because excretion curves of patients with NORMAL rates of emptying are typically declining at 180 – 240 minutes (as is the reference range derived from healthy subjects) and those of DELAYED patients are still rising at 180 and 240 minutes, kPCD values of MODERATELY DELAYED patients (Exhibit III) may rise through the cut-off points at 180 and 240 minutes. Nevertheless, the continuously rising curve with peak excretion occurring at 240 minutes is indicative of delayed emptying as verified by scintigraphy in the GEBT validation study.
Exhibit III: Subject with MODERATELY DELAYED Gastric Emptying
Gastroparesis is best identified by observing if the patient’s kPCD values at either the 90, 120 or 150 minute time points are below the respective COPs, and/or if the patient’s maximum kPCD value occurs at 240 minutes.