Systematically testing human HMBS missense variants to reveal mechanism and pathogenic variation.

Defects in hydroxymethylbilane synthase (HMBS) can cause acute intermittent porphyria (AIP), an acute neurological disease. Although sequencing-based diagnosis can be definitive, ∼⅓ of clinical HMBS variants are missense variants, and most clinically reported HMBS missense variants are designated as "variants of uncertain significance" (VUSs). Using saturation mutagenesis, en masse selection, and sequencing, we applied a multiplexed validated assay to both the erythroid-specific and ubiquitous isoforms of HMBS, obtaining confident functional impact scores for >84% of all possible amino acid substitutions. The resulting variant effect maps generally agreed with biochemical expectations and provide further evidence that HMBS can function as a monomer. Additionally, the maps implicated specific residues as having roles in active site dynamics, which was further supported by molecular dynamics simulations. Most importantly, these maps can help discriminate pathogenic from benign HMBS variants, proactively providing evidence even for yet-to-be-observed clinical missense variants.

Practical recommendations for biochemical and genetic diagnosis of the porphyrias.

The porphyrias are a group of rare inborn errors of metabolism associated with various clinical presentations and long-term complications, making them relevant differential diagnoses to consider for many clinical specialities, especially hepatologists, gastroenterologists and dermatologists. To diagnose a patient with porphyria requires appropriate biochemical investigations, as clinical features alone are not specific enough. Furthermore, it is important to be aware that abnormalities of porphyrin accumulation and excretion occur in many other disorders that are collectively far more common than the porphyrias. In this review, we provide an overview of porphyria-related tests with their strengths and limitations, give recommendations on requesting and diagnostic approaches in non-expert and expert laboratories for different clinical scenarios and discuss the role of genetic testing in the porphyrias. To diagnose porphyria in a currently symptomatic patient requires analysis of biochemical markers to demonstrate typical patterns of haem precursors in urine, faeces and blood. The use of genomic sequencing in diagnostic pathways for porphyrias requires careful consideration, and the demonstration of increased porphyrin-related markers is necessary prior to genomic testing in symptomatic patients. In the acute porphyrias, genomic testing is presently a useful adjunct for genetic counselling of asymptomatic family members and the most common cutaneous porphyria, porphyria cutanea tarda, is usually a sporadic, non-hereditary disease. Getting a correct and timely porphyria diagnosis is essential for delivering appropriate care and ensuring best patient outcome.

Congenital erythropoietic porphyria.

Congenital erythropoietic porphyria (CEP) is a rare autosomal recessive disease due to the deficient, but not absent, activity of uroporphyrinogen III synthase (UROS), the fourth enzyme in the heme biosynthesis pathway. Biallelic variants in the UROS gene result in decreased UROS enzymatic activity and the accumulation of non-physiologic Type I porphyrins in cells and fluids. Overproduced uroporphyrins in haematopoietic cells are released into the circulation and distributed to tissues, inducing primarily hematologic and dermatologic symptoms. The clinical manifestations vary in severity ranging from non-immune hydrops fetalis in utero to mild dermatologic manifestations in adults. Here, the biochemical, molecular and clinical features of CEP as well as current and new treatment options, including the rescue of UROS enzyme activity by chaperones, are presented.

ALAD Inhibition by Porphobilinogen Rationalizes the Accumulation of δ-Aminolevulinate in Acute Porphyrias.

Patients with major forms of acute hepatic porphyria present acute neurological attacks with overproduction of porphobilinogen (PBG) and δ-aminolevulinic acid (ALA). Even if ALA is considered the most likely agent inducing the acute symptoms, the mechanism of its accumulation has not been experimentally demonstrated. In the most frequent form, acute intermittent porphyria (AIP), inherited gene mutations induce a deficiency in PBG deaminase; thus, accumulation of the substrate PBG is biochemically obligated but not that of ALA. A similar scenario is observed in other forms of acute hepatic porphyria (i.e., porphyria variegate, VP) in which PBG deaminase is inhibited by metabolic intermediates. Here, we have investigated the molecular basis of δ-aminolevulinate accumulation using in vitro fluxomics monitored by NMR spectroscopy and other biophysical techniques. Our results show that porphobilinogen, the natural product of δ-aminolevulinate deaminase, effectively inhibits its anabolic enzyme at abnormally low concentrations. Structurally, this high affinity can be explained by the interactions that porphobilinogen generates with the active site, most of them shared with the substrate. Enzymatically, our flux analysis of an altered heme pathway demonstrates that a minimum accumulation of porphobilinogen will immediately trigger the accumulation of δ-aminolevulinate, a long-lasting observation in patients suffering from acute porphyrias.

Evaluation of Metabolic Changes in Acute Intermittent Porphyria Patients by Targeted Metabolomics.

Acute intermittent porphyria (AIP) is an inherited rare hepatic disorder due to mutations within the hydroxymethylbilane gene. AIP patients with active disease overproduce aminolevulinic acid (ALA) and porphobilinogen (PBG) in the liver which are exported inducing severe neurological attacks. Different hepatic metabolic abnormalities have been described to be associated with this condition. The goal of this research was to explore the metabolome of symptomatic AIP patients by state-of-the art liquid chromatography-tandem mass spectrometry (LC-MS/MS). A case versus control study including 18 symptomatic AIP patients and 33 healthy controls was performed. Plasmatic levels of 51 metabolites and 16 ratios belonging to four metabolic pathways were determined. The results showed that the AIP patients presented significant changes in the two main areas of the metabolome under study: (a) the tryptophan/kynurenine pathway with an increase of tryptophan in plasma together with increase of the kynurenine/tryptophan ratio; and (b) changes in the tricarboxylic acid cycle (TCA) including increase of succinic acid and decrease of the fumaric acid/succinic acid ratio. We performed a complementary in vitro study adding ALA to hepatocytes media that showed some of the effects on the TCA cycle were parallel to those observed in vivo. Our study confirms in plasma previous results obtained in urine showing that AIP patients present a moderate increase of the kynurenine/tryptophan ratio possibly associated with inflammation. In addition, it also reports changes in the mitochondrial TCA cycle that, despite requiring further research, could be associated with an energy misbalance due to sustained overproduction of heme-precursors in the liver.

Dysregulation of homocysteine homeostasis in acute intermittent porphyria patients receiving heme arginate or givosiran.

Acute intermittent porphyria (AIP) is a rare metabolic disease caused by mutations within the hydroxymethylbilane synthase gene. Previous studies have reported increased levels of plasma total homocysteine (tHcy) in symptomatic AIP patients. In this study, we present long-term data for tHcy and related parameters for an AIP patient cohort (n = 37) in different clinical disease-states. In total, 25 patients (68%) presented with hyperhomocysteinemia (HHcy; tHcy > 15 µmol/L) during the observation period. HHcy was more frequent in AIP patients with recurrent disease receiving heme arginate, than in nonrecurrent (median tHcy: 21.6 µmol/L; range: 10-129 vs median tHcy: 14.5 µmol/L; range 6-77). Long-term serial analyses showed a high within-person tHcy variation, especially among the recurrent patients (coefficient of variation: 16.4%-78.8%). HHcy was frequently associated with low blood concentrations of pyridoxal-5'-phosphate and folate, while cobalamin concentration and the allele distribution of the methylene-tetrahydrofolate-reductase gene were normal. Strikingly, 6 out of the 9 recurrent patients who were later included in a regime of givosiran, a small-interfering RNA that effectively reduced recurrent attacks, showed further increased tHcy (median tHcy in 9 patients: 105 µmol/L; range 16-212). Screening of amino acids in plasma by liquid-chromatography showed co-increased levels of methionine (median 71 µmol/L; range 23-616; normal <40), suggestive of acquired deficiency of cystathionine-ß-synthase. The kynunerine/tryptophan ratio in plasma was, however, normal, indicating a regular metabolism of tryptophan by heme-dependent enzymes. In conclusion, even if HHcy was observed in AIP patients receiving heme arginate, givosiran induced an aggravation of the dysregulation, causing a co-increase of tHcy and methionine resembling classic homocystinuria.

Evaluation of an immunoassay procedure to measure 6-monoacetylmorphine.

Introduction: 6-Monoacetylmorphine (6-MAM) is a specific metabolite of heroin. Thus, the presence of 6-MAM in urine is a definitive indication of heroin intake. The possibility of having an immunoassay procedure to measure 6-MAM would be a diagnosis tool to discriminate, among opiates-positive, those patients who have consumed heroin and those who have not.Methods: EMIT® II Plus 6-Acetylmorphine Assay was used to measure 6-MAM in urine. The positive opiate screening results were confirmed at the Toxicology laboratory of our hospital by GC-MS.Results: This study includes 63 urine samples from subjects admitted to emergency department with suspicion of opiate consumption. Specificity was evaluated in the two groups of samples studied. In the first group all samples which resulted negative by opiate immunoassay (n = 33) were negative for 6-MAM immunoassay test. Thus, the specificity obtained for 6-MAM immunoassay in this group was 100%. Regarding the second specificity study, performed in positive samples by opiate immunoassay which were negative to 6 MAM by GC-MS, the specificity decreased down to 75%. In the study of sensitivity all samples confirmed as positive to 6-MAM by confirmatory method (GC-MS) resulted positive by the screening method, thus sensitivity obtained was 100%.Discussion: In this study no FN for 6-MAM was observed and therefore the new Emit® II Plus 6- Acetylmorphine Assay procedure has a high NPV, thus a negative result with 6-MAM immunoassay practically excludes heroine consume. The positive results to 6-MAM by immunoassay should be confirmed by a more analytically specific method, such as GCMS.

International Porphyria Molecular Diagnostic Collaborative: an evidence-based database of verified pathogenic and benign variants for the porphyrias.

With the advent of precision and genomic medicine, a critical issue is whether a disease gene variant is pathogenic or benign. Such is the case for the three autosomal dominant acute hepatic porphyrias (AHPs), including acute intermittent porphyria, hereditary coproporphyria, and variegate porphyria, each resulting from the half-normal enzymatic activities of hydroxymethylbilane synthase, coproporphyrinogen oxidase, and protoporphyrinogen oxidase, respectively. To date, there is no public database that documents the likely pathogenicity of variants causing the porphyrias, and more specifically, the AHPs with biochemically and clinically verified information. Therefore, an international collaborative with the European Porphyria Network and the National Institutes of Health/National Center for Advancing Translational Sciences/National Institute of Diabetes and Digestive and Kidney Diseases (NIH/NCATS/NIDDK)-sponsored Porphyrias Consortium of porphyria diagnostic experts is establishing an online database that will collate biochemical and clinical evidence verifying the pathogenicity of the published and newly identified variants in the AHP-causing genes. The overall goal of the International Porphyria Molecular Diagnostic Collaborative is to determine the pathogenic and benign variants for all eight porphyrias. Here we describe the overall objectives and the initial efforts to validate pathogenic and benign variants in the respective heme biosynthetic genes causing the AHPs.

Association between hepatitis C virus and porphyria cutanea tarda.

Porphyria cutanea tarda (PCT) arises from a deficiency of uroporphyrinogen decarboxylase (UROD) in the liver. Several exogenous risk factors are associated with the acquired form of the disease. In Southern Europe, PCT is strongly linked to hepatitis C virus (HCV) infection to the point that a high prevalence of viral infection in some geographic areas generated an increase of PCT cases as a complication. In spite of the association, PCT is a rare complication of HCV infection, thus suggesting the existence of susceptibility factors operating in only some patients. Investigation of liver specimens of PCT patients showed iron accumulation, which albeit moderate, was higher in comparison with HCV-infected patients without PCT. Measurements of hepcidin in serum of HCV-infected patients with and without PCT and calculation of hepcidin/ferritin ratio were compatible with the hypothesis that HCV induced inadequate response of hepcidin to iron accumulation. Administration of direct-acting antivirals (DAA) to HCV-infected patients with active PCT showed that eradication of the virus was followed by resolution of PCT and rapid disappearance of urinary porphyrins. This suggests a direct participation of the virus in the oxidative mechanism leading to UROD inhibition. If clinical evolution of HCV- PCT-patients is placed within a time-frame, rapid PCT resolution by DAA is in striking contrast with a long-delay (in most cases of decades) between viral infection and appearance of overt porphyria. This could be explained if HCV infection (a): enhanced an oxidative environment in the vicinity of UROD and (b): facilitated iron accumulation through hepdicin down-regulation. Thus, only when iron accumulation reached a threshold, inhibition of UROD attained a critical level. However, the enigma is why only a minority of HCV-infected patients develop PCT. If additional risk factors (i.e. alcohol abuse) are not concurring, it should be concluded that modifier genes or epigenetic mechanisms related to iron homeostasis, facilitate iron progressive accumulation in only a minority susceptible patients.

Potential value of urinary amatoxin quantification in patients with hepatotoxic mushroom poisoning.

BACKGORUND & AIMS: Mushroom poisoning with Amanita phalloides or similar species can lead to liver failure with 10-30% mortality rates. We aimed at defining the prognostic value of urinary amatoxin quantification in patients with hepatotoxic mushroom poisoning. METHODS: Data from 32 patients with hepatotoxic mushroom poisoning (Hospital Clínic Barcelona, 2002-16) in whom urinary amatoxins were determined (ELISA) were retrospectively reviewed. Correlations between urinary amatoxin and collected baseline variables with outcomes including hepatotoxicity (ALT>1000 U/L), severe acute liver injury (ALI, prothrombin <50%), acute liver failure (ALF, ALI and encephalopathy), transplantation/death and hospital length-of-stay, were evaluated. RESULTS: 12/32 patients developed increased aminotransferase activity. Among the 13/32 amatoxin negative patients, 1 developed ALI and 12/13 no hepatotoxicity. Among the 19/32 amatoxin positive patients, 8/19 (42%) developed hepatotoxicity, including 5 who progressed to severe ALI, of whom 3 developed ALF (2 deaths, 1 transplantation). Urinary amatoxin and prothrombin were independent predictors of hepatotoxicity, ALT peak values (along with age) and hospital length-of-stay. In positive amatoxins patients, urinary concentrations > 55 ng/ml (or a baseline prothrombin ≤ 83%), were associated to hepatotoxicity (presented by 8/9 patients with ALT>1000 U/L). Among 5 patients with urinary amatoxin ≥ 70 ng/ml, 4 developed severe ALI. CONCLUSIONS: In patients with hepatotoxic mushroom poisoning, a negative urinary amatoxin quantification within 72h of intake ruled out the risk of hepatotoxicity in 92% of patients, whereas positive urinary amatoxins were associated with hepatotoxicity and severe ALI. Concentrations >55 ng/ml and ≥ 70 ng/ml were predictive of hepatotoxicity and severe ALI, respectively.

Human immunodeficiency virus and risk of porphyria cutanea tarda: a possible association examined in a large hospital.

BACKGROUND: Human immunodeficiency virus (HIV) infection has been reported to be a risk factor for porphyria cutanea tarda (PCT). PURPOSE: To assess the strength of the association between HIV infection and PCT in a large hospital setting. METHODS: All patients (N = 210) diagnosed of PCT between 1990 and 2014 were retrospectively investigated for HIV infection and co-precipitating factors. High-performance liquid chromatography was used to assess the appearance of pre-PCT urinary porphyrin abnormalities among a group (N = 22) of HIV-infected patients without PCT using the urine of patients co-infected with hepatitis C virus (HCV) without PCT for comparison. RESULTS: Twenty-six HIV-infected patients (19 males and seven females) were diagnosed of PCT. During the same interval of time, ~8000 different patients infected with HIV were attended in the hospital of infectious diseases department. Examination of risk factors showed that 25 out of 26 of the PCT patients with HIV were co-infected with HCV. No chromatographic abnormalities were found in the urine of non-PCT-HIV-infected patients, whereas 39% co-infected patients showed urinary porphyrin abnormalities. CONCLUSIONS: In our large hospital series, the appearance of PCT among HIV-infected patients is low (<1%) and most present co-infection with HCV. Therefore, in most HIV-infected patients with PCT, hepatitis C and not HIV may induce uroporphyrinogen decarboxylase deficiency.

ALAS2 acts as a modifier gene in patients with congenital erythropoietic porphyria.

Mutations in the uroporphyrinogen III synthase (UROS) gene cause congenital erythropoietic porphyria (CEP), an autosomal-recessive inborn error of erythroid heme biosynthesis. Clinical features of CEP include dermatologic and hematologic abnormalities of variable severity. The discovery of a new type of erythroid porphyria, X-linked dominant protoporphyria (XLDPP), which results from increased activity of 5-aminolevulinate synthase 2 (ALAS2), the rate-controlling enzyme of erythroid heme synthesis, led us to hypothesize that the CEP phenotype may be modulated by sequence variations in the ALAS2 gene. We genotyped ALAS2 in 4 unrelated CEP patients exhibiting the same C73R/P248Q UROS genotype. The most severe of the CEP patients, a young girl, proved to be heterozygous for a novel ALAS2 mutation: c.1757 A > T in exon 11. This mutation is predicted to affect the highly conserved and penultimate C-terminal amino acid of ALAS2 (Y586). The rate of 5-aminolevulinate release from Y586F was significantly increased over that of wild-type ALAS2. The contribution of the ALAS2 gain-of-function mutation to the CEP phenotype underscores the importance of modifier genes underlying CEP. We propose that ALAS2 gene mutations should be considered not only as causative of X-linked sideroblastic anemia (XLSA) and XLDPP but may also modulate gene function in other erythropoietic disorders.

Successful treatment of congenital erythropoietic porphyria using matched unrelated hematopoietic stem cell transplantation.

Congenital erythropoietic porphyria (CEP), or Günther's disease, is an inborn error of metabolism produced by a deficiency of uroporphyrinogen III synthase (UROS), the fourth enzyme of the heme biosynthesis pathway. This enzymatic defect induces the accumulation of isomer I porphyrins in erythrocytes, skin, and tissues, producing various clinical manifestations. Severe cases are characterized by extreme photosensitivity, causing scarring and mutilations, and by hemolytic anemia, reducing life expectancy. CEP is caused by mutations in the UROS gene, and one of the most severe forms of the disease is associated with a cysteine to arginine substitution at residue 73 of the protein (C73R). CEP has been successfully treated only by the transplantation of hematopoietic precursors. We report the case of a male infant with severe postdelivery symptoms diagnosed with CEP and found to be homozygous for the C73R mutation. He underwent successful allogeneic bone marrow transplantation from a matched unrelated donor at 7 months of age. The hemolytic anemia was corrected and the porphyrin overproduction was significantly reduced. The patient remained asymptomatic after 1 year. This new case confirms that patients with severe CEP can benefit from early postnatal hematopoietic stem cell transplantation.

Transcriptomic study in explanted liver from a patient with acute intermittent porphyria.

Acute intermittent porphyria (AIP) is a rare disease caused by a deficiency of hydroxymethylbilane synthase (HMBS), the third enzyme of the heme-synthesis pathway. Decreased enzymatic activity in the liver induces an overproduction of heme-precursors and acute neurological attacks. We report a 36-years-old female with AIP with a long-term history of severe, disabling, recurrent attacks, who underwent curative liver transplantation. Tissue samples from the explant were obtained for transcriptome analysis. Whole RNA was extracted and 16 gene-transcripts were selected and investigated by quantitative polymerase chain reaction. These included nine genes encoding enzymes that consecutively catalyze heme-synthesis and catabolism in the liver (ALAS1; ALAD; HMBS; UROS; UROD; CPOX; PPOX; FECH; HMOX1). Additionally, we studied genes related to inflammation (IL6; TNF) insulin signaling (PGC-1α; IGF-1; FOXO-1) and tryptophan metabolism (TDO2; IDO). Transcripts of eight house-keeping genes were co-measured for normalization. All transcripts were also measured in five control samples from healthy living liver donors. The transcriptome of the controls showed important differences between the various genes, with the first two genes of the heme-synthesis pathway, ALAS1 and ALAD showing strikingly high mRNA levels compared to the consecutive HMBS gene. Transcripts of several genes significantly differed in the AIP liver compared to controls. Transcripts of HMOX1 and UROS were increased in the AIP liver whereas transcripts of UROD; CPOX, PPOX, and TDO2 were decreased. ALAS1 expression was not increased, possibly due to hemin administered to the patient before transplantation. These results highlight several transcriptomic changes related to heme homeostasis in AIP.

Systematically testing human HMBS missense variants to reveal mechanism and pathogenic variation.

Defects in hydroxymethylbilane synthase (HMBS) can cause Acute Intermittent Porphyria (AIP), an acute neurological disease. Although sequencing-based diagnosis can be definitive, ~⅓ of clinical HMBS variants are missense variants, and most clinically-reported HMBS missense variants are designated as "variants of uncertain significance" (VUS). Using saturation mutagenesis, en masse selection, and sequencing, we applied a multiplexed validated assay to both the erythroid-specific and ubiquitous isoforms of HMBS, obtaining confident functional impact scores for >84% of all possible amino-acid substitutions. The resulting variant effect maps generally agreed with biochemical expectation. However, the maps showed variants at the dimerization interface to be unexpectedly well tolerated, and suggested residue roles in active site dynamics that were supported by molecular dynamics simulations. Most importantly, these HMBS variant effect maps can help discriminate pathogenic from benign variants, proactively providing evidence even for yet-to-be-observed clinical missense variants.

Brainstem dysfunction in variegate porphyria.

INTRODUCTION: Variegate porphyria (VP) is a rare metabolic disorder that may present as an acute predominantly motor neuropathy. Cranial nerves and brainstem functions have been only scarcely studied. METHODS: Brainstem reflexes were examined in symptomatic and non-symptomatic VP mutation carriers of a single family. RESULTS: Similar results were found in the 2 patients with a history of porphyric crises. The blink reflex showed an absence of late responses (R2 and R2c) to stimulation of both sides. The masseter inhibitory reflex showed reduced inhibition of the second phase. The jaw jerk was normal. The asymptomatic carriers did not show any of the abnormalities just noted. CONCLUSIONS: Our results are compatible with a central lower pons-upper medulla disorder in the brainstem. We hypothesize that brainstem dysfunction in VP patients with a history of porphyric crises may be due to neurotoxic effects of porphyrin precursors as well as subclinical osmolarity changes due to hyponatremia.

Phototolerance induced by narrow-band UVB phototherapy in severe erythropoietic protoporphyria.

Erythropoietic protoporphyria arises from an inherited disorder of porphyrin metabolism which leads to an accumulation of protoporphyrin IX in the erythropoietic system and other tissues. It is characterized by cutaneous photosensitivity, usually difficult to keep under control. Among the scant therapeutic options proposed to reduce photosensitivity in erythropoietic protoporphyria, narrow-band UVB phototherapy has occasionally been used to induce sunlight tolerance. We report an adult case of erythropoietic protoporphyria with a severe photosensitivity treated with narrow-band UVB that developed an appropriate sunlight phototolerance, without adverse events during phototherapy.

European specialist porphyria laboratories: diagnostic strategies, analytical quality, clinical interpretation, and reporting as assessed by an external quality assurance program.

BACKGROUND: The porphyrias are a group of rare metabolic disorders whose diagnosis depends on identification of specific patterns of porphyrin precursor and porphyrin accumulation in urine, blood, and feces. Diagnostic tests for porphyria are performed by specialized laboratories in many countries. Data regarding the analytical and diagnostic performance of these laboratories are scarce. METHODS: We distributed 5 sets of multispecimen samples from different porphyria patients accompanied by clinical case histories to 18-21 European specialist porphyria laboratories/centers as part of a European Porphyria Network organized external analytical and postanalytical quality assessment (EQA) program. The laboratories stated which analyses they would normally have performed given the case histories and reported results of all porphyria-related analyses available, interpretative comments, and diagnoses. RESULTS: Reported diagnostic strategies initially showed considerable diversity, but the number of laboratories applying adequate diagnostic strategies increased during the study period. We found an average interlaboratory CV of 50% (range 12%-152%) for analytes in absolute concentrations. Result normalization by forming ratios to the upper reference limits did not reduce this variation. Sixty-five percent of reported results were within biological variation-based analytical quality specifications. Clinical interpretation of the obtained analytical results was accurate, and most laboratories established the correct diagnosis in all distributions. CONCLUSIONS: Based on a case-based EQA scheme, variations were apparent in analytical and diagnostic performance between European specialist porphyria laboratories. Our findings reinforce the use of EQA schemes as an essential tool to assess both analytical and diagnostic processes and thereby to improve patient care in rare diseases.

Tetracaine from urethral ointment causes false positive amphetamine results by immunoassay.

BACKGROUND: Amphetamine urine drug screening by immunoassay is prone to cross-react with other compounds leading to false positive results. Tetracaine is a local anesthetic drug used in the clinical setting as an ointment during urinary catheterization. In our laboratory, tetracaine is often detected by gas chromatography-mass spectrometry in the urine of patients admitted in the emergency department with false positive amphetamine results. The objectives of this study were to investigate if there was cross-reactivity to tetracaine in an amphetamine immunoassay and to retrospectively evaluate the potential contribution of tetracaine to false positive amphetamine results. METHODS: An interference study was conducted using negative urine samples spiked with increasing concentrations of tetracaine hydrochloride and analyzed with the CEDIA Amphetamine/Ecstasy immunoassay. Retrospectively, urine samples of patients which yielded positive amphetamine immunoassay results and were analyzed by gas chromatography-mass spectrometry were reviewed (n = 417). The presence of tetracaine and/or other drugs by gas chromatography-mass spectrometry were gathered. RESULTS: Tetracaine caused false positive amphetamine results by immunoassay (cut-off 1000 µg/L) with a concentration of above 40 mg/L. Retrospective analysis of all positive amphetamine immunoassay samples showed that in 45 out of the 417 (10.8%) urine samples no amphetamine-like derivative was identified by gas chromatography - mass spectrometry. In 37 out of 45 (82.2%) of these false positive cases tetracaine was detected, of whom 59.5% (22/37) had an estimated tetracaine concentration of ≥40 mg/L. CONCLUSIONS: This study confirmed the interference of tetracaine in the CEDIA Amphetamine/Ecstasy immunoassay and that tetracaine may have contributed to around 80% of the false positive amphetamine cases in the urine samples of patients admitted to the emergency department at our institution.