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Clinical Chemistry 50: 915-923, 2004. First published February 19, 2004; 10.1373/clinchem.2003.025213
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(Clinical Chemistry. 2004;50:915-923.)
© 2004 American Association for Clinical Chemistry, Inc.

General Clinical Chemistry

Plasma Fluorescence Scanning and Fecal Porphyrin Analysis for the Diagnosis of Variegate Porphyria: Precise Determination of Sensitivity and Specificity with Detection of Protoporphyrinogen Oxidase Mutations as a Reference Standard

Richard J. Hift1,a, Brandon P. Davidson1, Cornelis van der Hooft1, Doreen M. Meissner1 and Peter N. Meissner1

1 Lennox Eales Porphyria Laboratories, Medical Research Council/University of Cape Town Liver Research Centre and Department of Medicine, University of Cape Town Faculty of Health Sciences, Observatory, South Africa.

aAddress correspondence to this author at: Department of Medicine, Medical School, Anzio Road, Observatory, South Africa 7925. Fax 27-21-447-9670; e-mail rjh{at}liver.uct.ac.za.

Background: Variegate porphyria (VP) is the autosomal dominant disorder associated with deficiency of the enzyme protoporphyrinogen oxidase (PPOX). Plasma fluorescence scanning has been reported to be a more sensitive test for VP than traditional fecal chromatography. Previous comparisons of these techniques predated identification of the PPOX gene. We assessed these techniques in a large group of patients characterized for VP at the DNA level.

Methods: We evaluated all patients for whom the genotype and a plasma scan or fecal porphyrin result were available. Mutations were detected by restriction digest analysis. Plasma fluorescence scanning was conducted according to published methods. Fecal porphyrins were identified and quantified by thin-layer chromatography.

Results: Plasma fluorescence scanning was assessed in 679 patients (205 with VP who were carriers of a PPOX mutation, either with disease symptoms or asymptomatic) and fecal analysis in 473 (190 with VP). Sensitivity and specificity of both tests were higher in adults than in children and higher for adults with disease symptoms than for asymptomatic carriers. In a direct comparison in 168 adults (73 with VP), plasma scanning was significantly more sensitive than fecal porphyrin analysis [sensitivity, 0.96 (95% confidence interval, 0.89–0.99) vs 0.77 (0.66–0.85)]. Fecal coproporphyrin [area under the curve, 0.87 (0.83–0.90)] was a better predictor of VP than protoporphyrin [0.80 (0.76–0.84)].

Conclusions: Plasma scanning is a more sensitive and specific test for VP than fecal porphyrin analysis. Neither test is sensitive in children, and both are less sensitive in asymptomatic carriers than in symptomatic cases. DNA analysis therefore remains the preferred method for the identification of carriers, particularly in children.




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