Exact profiles of (13)CO(2) recovery in breath air after per oral administration of [(13)C]methacetin in two groups of different ages

13C Methacetin Breath Test for Assessment of Microsomal Liver Function: Methodology and Clinical Application

Assessment of the liver function, and the need of constant monitoring of the organ's capacity, concerns not only patients with primary liver diseases, but also those at risk of hepatopathies secondary to other chronic diseases. Most commonly, the diagnostics is based on measurements of static biochemical parameters, which allow us to draw conclusions only indirectly about the function and the degree of damage of the organ. On the other hand, liver biopsy is an invasive procedure and therefore it is associated with a considerable risk of complications. Dynamic tests enable us to assess quantitatively the organ's functional reserve by analyzing the kinetics of the metabolization of the substrate by the liver. In practice applied are breath tests using substances such as aminopyrine, caffeine, methacetin, erythromycin (for assessment of the microsomal function); phenylalanine, galactose (for assessment of the cytosolic function); methionine, octanoate, ketoisocaproic acid (for assessment of the mitochondrial function). The test with ¹³C methacetin belongs to the best described and most widely applied methods in noninvasive liver function assessment. Due to the rising availability of this method, knowledge concerning its limitations and controversies regarding the methodology, as well as its usefulness in chosen groups of patients, seems to be vital.

(13)CO2 breath tests in non-invasive hepatological diagnosis

In liver diagnostics, a simple, non-invasive test with high sensitivity and specificity is permanently being sought in order to assess the degree of liver damage. In addition to liver biopsy, algorithms using blood parameters or elastometry are used in clinical practice. However, these methods do not provide information about the true liver reserve, so the liver breath test seem to be a promising diagnostic tool. The basis of this test depends on the ability of particular hepatocyte enzyme systems to metabolise a tested substance labelled with a stable carbon isotope. The kinetics of ¹³CO₂ elimination with expiratory air then permits quantitative assessment of the functional liver reserve and the degree of organ damage. In this paper the most commonly used tests, grouped according to the main metabolic pathways, are described. The usefulness of liver breath tests in specific clinical situations, both as a diagnostic and prognostic tool, is presented.

Impact of combined oral contraceptives containing ethinylestradiol on the liver microsomal metabolism

OBJECTIVES

To check whether currently used combined oral contraceptives (COCs) containing ethinylestradiol (EE) affect the liver microsomal metabolism.

METHODS

(13)C-methacetin breath test ((13)C-MBT) - a sensitive non-invasive probe of cytochrome P-450 1A2 activity - was performed in 15 women on day 14, 15, 16, 17 or 18 of intake of their COC (containing EE), and between day 1 and 5 during the withdrawal bleeding, as well as in nine women not using hormonal contraception during the luteal phase of their cycle (between the 17th and the 23rd day), and between day 1 and 5 during menstruation.

RESULTS

The maximum breath (13)C elimination was significantly lower during the phase of intake of contraceptive pills than during withdrawal bleeding: 31.5 ± 2.2 %/h vs. 38.2 ± 1.9 %/h (p = 0.0045), whereas the time to reach it was similar on the two study days: 21.2 ± 1.2 min vs. 21.0 ± 1.1 min. Between the 27th and the 180th min of observation the cumulative breath (13)C elimination was statistically significantly lower during intake of the pill than during withdrawal bleeding. No significant menstrual cycle phase-dependent fluctuations in the results of the (13)C- methacetin breath test were observed in the control group.

CONCLUSION

COCs containing EE markedly inhibit hepatic microsomal function. This phenomenon must be taken into consideration when interpreting results of (13)C-MBT.

Interpretation of non-invasive breath tests using (13)C-labeled substrates--a preliminary report with (13)C-methacetin

Non-invasive breath tests can serve as valuable diagnostic tools in medicine as they can determine particular enzymatic and metabolic functions in vivo. However, methodological pitfalls have limited the actual clinical application of those tests till today. A major challenge of non-invasive breath tests has remained the provision of individually reliable test results. To overcome these limitations, a better understanding of breath kinetics during non-invasive breaths tests is essential. This analysis compares the breath recovery of a ¹³C-methacetin breath test with the actual serum kinetics of the substrate. It is shown, that breath and serum kinetics of the same test are significantly different over a period of 60 minutes. The recovery of the tracer ¹³CO₂ in breath seems to be significantly delayed due to intermediate storage in the bicarbonate pool. This has to be taken into account for the application of non-invasive breath test protocols. Otherwise, breath tests might display bicarbonate kinetics despite the metabolic capacity of the particular target enzyme.

Comparison of two dosage regimens of the substrate for the [13C]methacetin breath test

The [(13)C]methacetin breath test ([(13)C]MBT)--a valuable non-invasive tool dedicated to the assessment of the liver metabolic capacity--still needs standardisation. The aim of this study was to check whether currently used dosage regimens of [(13)C]methacetin provide concordant [(13)C]MBT results in subjects with an atypical body constitution. Healthy volunteers: low body mass<55 kg (eight women), and high body mass>95 kg (eight large body frame men) were recruited. They underwent [(13)C]MBT on separate days, taking in random order [(13)C]methacetin: a fixed 75 mg dose (FX75), or a 1 mg kg(-1) body mass-adjusted dose (BMAD). Samples of expiratory air for (13)CO(2) measurement were collected over 3 h. The maximum momentary (13)C elimination in breath air occurred earlier and was higher following BMAD than with FX75 in the low body mass females (T (max) 14.6 ± 1.0 min vs. 22.1 ± 2.4 min, p = 0.019; D (max) 41.9 ± 2.9 % dose h(-1) vs. 36.6 ± 3.6 % dose h(-1), p = 0.071). In the high body mass men, T (max) remained unchanged, whereas D (max) was slightly higher with BMAD compared to FX75 (21.5 ± 3.2 min vs. 23.0 ± 3.0 min; 38.5 ± 2.9 % dose h(-1) vs. 32.3 ± 2.5 % dose h(-1)). It is concluded that in subjects with a body constitution outside the general population average, the dosage of the substrate may affect some results of the [(13)C]MBT. The dosage-related differences appear, however, to be insignificant if the result of the [(13)C]MBT is reported as a cumulative (13)C recovery in breath air.

13C-methacetin breath test reproducibility study reveals persistent CYP1A2 stimulation on repeat examinations

AIM: To find the most reproducible quantitative parameter of a standard ¹³C-methacetin breath test (¹³C-MBT).

METHODS: Twenty healthy volunteers (10 female, 10 male) underwent the ¹³C-MBT after intake of 75 mg ¹³C-methacetin p.o. on three occasions. Short- and medium-term reproducibility was assessed with paired examinations taken at an interval of 2 and 18 d (medians), respectively.

RESULTS: The reproducibility of the 1-h cumulative ¹³C recovery (AUC_0-60), characterized by a coefficient of variation of 10%, appeared to be considerably better than the reproducibility of the maximum momentary ¹³C recovery or the time of reaching it. Remarkably, as opposed to the short gap between consecutive examinations, the capacity of the liver to handle ¹³C-methacetin increased slightly but statistically significantly when a repeat dose was administered after two to three weeks. Regarding the AUC_0-60, the magnitude of this fixed bias amounted to 7.5%. Neither the time gap between the repeat examinations nor the gender of the subjects affected the ¹³C-MBT reproducibility.

CONCLUSION: ¹³C-MBT is most reproducibly quantified by the cumulative ¹³C recovery, but the exactitude thereof may be modestly affected by persistent stimulation of CYP1A2 on repeat examinations.

Comparison of non-invasive assessment to diagnose liver fibrosis in chronic hepatitis B and C patients

OBJECTIVE

Chronic viral hepatitis B and C cause liver fibrosis, leading to cirrhosis. Fibrosis assessment is essential to establish prognosis and treatment indication. We compared seven non-invasive tests, separately and in combination, in chronic hepatitis patients to detect early stages of fibrosis according to the Metavir score in liver biopsy.

MATERIAL AND METHODS

Galactose and methacetin breath tests (GBT and MBT), biomarkers (hyaluronic acid (HA), aspartate aminotransferase platelet ratio index (APRI), FibroTest, and Fib-4) and transient elastography (TE) were evaluated in 89 patients. Additionally, 31 healthy controls were included for evaluation of breath tests and biomarkers.

RESULTS

Serum markers (HA, APRI, FibroTest, and Fib-4) and elastography significantly distinguished non-cirrhotic (F0123) from cirrhotic (F4) patients (p < 0.001, p = 0.015, p < 0.001, p = 0.005, p = 0.006, respectively). GBT, HA, APRI, FibroTest, Fib-4, and TE detected F01 from F234 (p = 0.04, p = 0.011, p = 0.009, p < 0.001, p < 0.001, and p < 0.001, respectively). A combination of different tests (TE, HA, and FibroTest) improved the performance statistically, area under the curve (AUC) = 0.87 for F234, 0.92 for F34, and 0.90 for F4.

CONCLUSION

HA, APRI, FibroTest, Fib-4, and TE reliably distinguish non-cirrhotic and cirrhotic patients. Except for MBT, all tests discriminate between mild and moderate fibrosis. As single tests: FibroTest, Fib-4, and TE were the most accurate for detecting early fibrosis; combining different non-invasive tests increased the accuracy for detection of liver fibrosis to such an extent and thus might be acceptable to replace liver biopsy.

Interference of acute cigarette smoking with [¹³C]methacetin breath test

It is essential to establish whether and how environmental factors affect the reliability of [(13)C]methacetin breath test ((13)C-MBT). In 12 healthy volunteers (smokers), a standard (13)C-MBT with 75 mg [(13)C]methacetin was performed twice in random order: on a control day without smoking and on another day with smoking two cigarettes antecedently. A considerable flattening of the curve of the momentary (13)C recovery within the expiratory air was observed when the (13)C-MBT was performed after smoking. The maximum of the momentary (13)C recovery, D(max), decreased from 37.20±2.58 to 25.39±2.29% dose/h (p=0.00052). Moreover, the time to reach D(max) was prolonged after cigarette smoking (26.5±3.1 vs. 16.5±1.9 min, p=0.0199). The curve of the cumulative (13)C recovery on the cigarette smoking day appeared to be shifted downwards, and statistically significant differences relative to the control situation were found between the 24th and 75th minute following [(13)C]methacetin administration. Smoking cigarettes immediately prior to the (13)C-MBT diminishes the ability of the liver to handle methacetin, and hence a possibility of such an interaction should be excluded in order to interpret the results of the test correctly.