When are metabolic ratios superior to absolute quantification? A statistical analysis.

Metabolite levels measured using magnetic resonance spectroscopy (MRS) are often expressed as ratios rather than absolute concentrations. However, the inter-subject variability of the denominator metabolite can introduce uncertainty into a metabolite ratio. In a clinical setting, there are no guidelines on whether ratios or absolute quantification should be used for a more accurate classification of normal versus abnormal results based on their statistical properties. In a research setting, the choice of one over the other can have significant implications on sample size, which must be factored in at the study design stage. Herein, we derive the probability distribution function for the ratio of two normally distributed random variables, and present analytical expressions for the comparison of ratios with absolute quantification in terms of both sample size and area under the receiver operator characteristic curve. The two approaches are compared for typical metabolite values found in the literature, and their respective merits are illustrated using previously acquired clinical MRS data in two pathologies: mild traumatic brain injury and multiple sclerosis. Our analysis shows that the decision between ratios and absolute quantification is non-trivial: in some cases, ratios might offer a reduction in sample size, whereas, in others, absolute quantification might prove more desirable for individual (i.e. clinical) use. The decision is straightforward and exact guidelines are provided in the text, given that population means and standard deviations of numerator and denominator can be reliably estimated.

Morbidity and mortality following transarterial liver chemoembolization in patients with hepatocellular carcinoma and synthetic hepatic dysfunction.

The purpose of this study was to determine the rate and risk factors for the development of irreversible hepatotoxicity after transarterial chemoembolization (TACE) in patients with hepatocellular carcinoma (HCC) and synthetic hepatic dysfunction. Two hundred fifty-one consecutive patients with HCC and hepatic dysfunction who underwent 443 TACE procedures from 2005 to 2010 were retrospectively reviewed. The included patients met one of the following criteria: a pre-TACE bilirubin level ≥ 2 mg/dL, an international normalized ratio (INR) > 1.5, a creatinine level > 1.2 mg/dL, a platelet count ≤ 60,000/mL, a Model for End-Stage Liver Disease (MELD) score > 15, Child-Turcotte-Pugh class B or C, ascites, or portal vein thrombosis. Hepatotoxicity was defined as new or worsening ascites, encephalopathy, or grade 3 or 4 toxicity (bilirubin, aspartate aminotransferase, alanine aminotransferase, creatinine, or INR) according to the National Cancer Institute Common Terminology Criteria for Adverse Events. The rate and risk factors for death or urgent liver transplantation within 6 weeks of TACE and irreversible hepatotoxicity were determined with a generalized estimating equation analysis. Reversible hepatotoxicity developed after 90 procedures (20%) in 78 patients (31%). Irreversible hepatotoxicity developed after 41 procedures (9%) in 37 patients (15%). Six patients (2%) underwent urgent liver transplantation, and 11 (4%) died within 6 weeks of TACE. Patients at increased risk for procedure-related mortality or urgent liver transplantation within 6 weeks of TACE had a baseline serum bilirubin level ≥ 4.0 mg/dL (P = 0.01), an elevated INR (P < 0.001), hypoalbuminemia with an albumin level < 2.0 g/L (P = 0.01), a serum creatinine level > 2.0 mg/dL (P = 0.02), large ascites (P = 0.002), encephalopathy (P = 0.005), or a MELD score ≥ 20 (P < 0.001). In conclusion, TACE can be performed safely in patients with baseline hepatic dysfunction. However, a poor hepatic reserve increases the risk of irreversible hepatotoxicity, which may lead to death or the need for urgent liver transplantation.