Number Of False-positive Results Research Articles

Case‐Control Association Tests Correcting for Population Stratification

In case-control association studies unobserved population stratification may act as a confounder, leading to an increased number of false positive results. Methods accounting for population structure by using additional genetic markers broadly follow one of two concepts: Genomic Control (GC) and Structured Association (SA). While extending existing methods of Structured Association we show that it is necessary to incorporate phenotypic information when inferring population structure, otherwise a systematic bias is introduced. Moreover, for moderate population stratification a Wald test statistic should be preferred as a Structured Association test statistic in comparison to a likelihood ratio test. The introduced extensions are compared to existing methods of Structured Association, as well as to Genomic Control, in a simulation study which is based on realistic situations of large case-control studies with moderate population stratification. A disadvantage of Genomic Control turns out to be the large variation in estimating the variance inflation factor, as well as the power loss if population structure increases. We come to the overall conclusion that Structured Association, if applied correctly, is superior to Genomic Control, at least in the case of simple population structure as simulated here.

Integrating epidemiology and genetic association: the challenge of gene-environment interaction.

Recent advances in human genomics have made it possible to better understand the genetic basis of disease. In addition, genetic association studies can also elucidate the mechanisms by which "non-genetic" exogenous and endogenous exposures influence the risk of disease. This is true both of studies that assess the marginal effect of a single gene and studies that look at the joint effect of genes and environmental exposures. For example, gene variants that are known to alter enzyme function or level can serve as surrogates for long-term biomarker levels that are impractical or impossible to measure on many subjects. Evidence that genetic variants modify the effect of an established risk factor may help specify the risk factor's biologically active components. We illustrate these ideas with several examples and discuss design and analysis challenges, particularly for studies of gene-environment interaction. We argue that to increase the power to detect interaction effects and limit the number of false positive results, large sample sizes will be needed, which are currently only available through planned collaborative efforts. Such collaborations also ensure a common approach to measuring variation at a genetic locus, avoiding a problem that has led to difficulties when comparing results from genetic association studies.

Computer-aided Lung Nodule Detection in CT: Results of Large-Scale Observer Test 1

The objective is to study the incremental effects of using a computer-aided lung nodule detection (CAD) system on the performance of a large pool of observers. A set of eight thin-section computed tomographic data sets with limited longitudinal coverage, containing a total of 22 lung nodules, was analyzed by using the automated nodule detection system. When applied to all eight cases, the CAD system alone achieved a detection rate of 86.4%, with 2.64 false-positive results per case. This study included 202 observers at a national radiology meeting: 39 thoracic radiologists, 95 non-thoracic radiologists, and 68 non-radiologists. Each participant read from one to eight cases in random order, first without and then with CAD system output available. Observer performance in nodule detection was measured before and after CAD was made available. Differences in performance of groups of observers before and after CAD were tabulated by mean, median, and SD in detection rate and number of false-positive results and tested by using nonparametric methods. In an analysis involving only the first randomly selected case read by all 202 participants, there were statistically significant increases in nodule detection rates and numbers of false-positive results for all types of observers. There was a significant difference in detection rates between radiologists and non-radiologists before CAD, but after CAD, there was no significant difference in detection rates between these observer types. In a second analysis involving 13 participants who read all eight cases, mean detection rates were 64.0% before CAD and 81.9% after CAD. Mean numbers of false-positive results were 0.144 per case before CAD and 0.173 after CAD. In a large observer study, use of a CAD system for nodule detection resulted in an incremental increase in detection rate, but also led to an increase in number of false-positive results. Also, CAD appears to be an equalizer of detection rates between observers of different levels of experience.

Poor predictive value of broad-range PCR for the detection of arthroplasty infection in 92 cases

Background The diagnosis of prosthetic infection remains a challenge, as no test is 100% sensitive and 100% specific Recent advances in molecular biology have enabled the detection of infection in culture negative cases.Patients and methods We evaluated the effectiveness of polymerase chain reaction (PCR) in detecting infection in failed joint replacements prospectively in 91 consecutive patients (92 prosthetic joints) undergoing revision total hip or knee arthroplasty. Synovial fluid was collected intraoperatively and examined by broad-range PCR assay for detection of bacterial DNA. The clinical diagnosis of infection was based on the results of blood tests, preoperative joint aspiration, culture and histology of multiple intraoperative tissue samples, as well as the surgeon's assessment.12 joints (13%) were infected, but the PCR was positive in 32 cases. The sensitivity of the technique was 92%, the specificity 74%, the accuracy 76%, the positive predictive value 34%, and the negative predictive value was 98%.Interpretation The PCR technique cannot be recommended for the routine detection of prosthetic infection. The large number of false positive results may represent sample contamination, or bacterial presence related to low-virulence organisms, low bacterial load, or a strong host immune response.

The possibilities of age reference values of PSA density in prostatic cancer screening

The possibilities of PSA (prostate specific antigen) test in screening of prostatic cancer may be evaluated by measuring its sensitivity, specificity and positive predictive value. The majority of reference articles in this field has evaluated the possibility of detection of prostatic cancer by radioimmunoassay (RIA). Prospective study included 2000 male patients over 50 years of age. The objective of the study was to examine the possibility of enzyme PSA test for differentiation of benign prostatic hyperplasia (BPH) and localized prostatic cancer (PC). The possibility of enzyme PSA assay for detection of prostatic cancer was compared with RIA method, by digital rectal examination and echotomography. The effect of PSA density and age reference values for differentiation of PC from BPH was also examined. The results of enzyme immunoassay test (EIA) were compatible with radioimmunoassay results (RIA). Diagnostic algorithm of prostatic cancer detection should be corrected with PSA density values according to aged-specific groups (49 years - 0.09 ng/ml/cm3; 59 years - 0.13 ng/ml/cm3; 69 years - 0.17 ng/ml/cm3; 79 years - 0.19 ng/ml/cm3) in order to reduce the number of false positive results in PSAD grey zone (0.10 - 0.20).

Reevaluation of growth hormone deficiency during and after growth hormone (GH) treatment: diagnostic value of GH tests and IGF-I and IGFBP-3 measurements.

Retesting of patients with growth hormone (GH) deficiency (GHD), especially those with idiopathic GHD, has yielded normalization of the results in several studies. The aim of this study was to reevaluate patients diagnosed as GHD at completion or reconfirm the diagnosis before completion of GH treatment by retesting with provocative tests, and to evaluate the value of IGF-I and IGFBP-3 levels in the diagnosis of GHD. Fifty (33 M, 17 F) patients with GHD (peak GH level <0.46 pmol/l (10 ng/ml]) in two pharmacological tests were retested and IGF-I and IGFBP-3 levels measured. The age of the patients at retest was 15.2+/-5.0 yr. Thirteen of 50 patients (26%) normalized their GH secretion. According to the initial diagnosis, 69% of those with partial GHD (peak GH level 0.32-0.46 pmol/l [7-10 ng/ml]), 43% with isolated GHD, 33% idiopathic and 11% of those with complete GHD (peak GH level <0.32 pmol/l [7 ng/ml]) normalized their GH level at retesting. None of the patients with multiple hormone deficiency and none with small pituitary on MRI normalized GH levels at retest. The sensitivities of IGF-I and of IGFBP-3 were 70% and 67%, respectively, and the specificities were 100%, when peak GH cutoff is taken as 0.46 pmol/l (10 ng/ml) for the diagnosis of GHD. The sensitivities of IGF-I and IGFBP-3 increased to 76.5% and 73.5% when the cutoff level for GHD is taken as 0.32 pmol/l (7 ng/ml). Those patients who normalized their GH levels at retest showed a satisfactory height velocity when GH therapy was discontinued. In conclusion, reevaluation of GH status may also be undertaken while patients are still on treatment as well as at completion of treatment, especially in patients with idiopathic, partial and isolated GHD, by retesting and by IGF-I and IGFBP-3 measurements. Lowering the cutoff level of GH peak at pharmacological tests to 0.32 pmol/l (7 ng/ml) will lower the number of false positive results in the diagnosis of GHD.

Specificity of three ELISA-gE kits for screening pig meat for antibodies to Aujeszky's disease

Muscle samples (20 g) from 2025 pig carcases from Aujeszky's disease-free holdings were collected at the slaughterhouse. The samples were frozen and thawed to obtain meat juice, which was then...

FALSE POSITIVE RESULTS IN GYNECOLOGIC ULTRASOUND DIAGNOSTICS

Background. Ultrasound examination is the most frequenty used and usually the only diagnostic imaging method in gynecology. It gives us informations about characteristics of pelvic organs and their changes and also enables us to form criteria that can be used in their evaluation. False positive results lead to unnecessary invasive diagnostic procedures. Methods. We give a review of possibilities modern ultrasonographic technology provides in evaluation of endometrium in postmenopause, adnexal masses and diagnostics of congenital uterine anomalies. Conclusions. Use of modern ultrasonographic technology reduces the number of false positive results.

In vivo autofluorescence imaging of early cancers in the human tracheobronchial tree with a spectrally optimized system.

The changes in the autofluorescence characteristics of the bronchial tissue is of crucial interest as a cancer diagnostic tool. Evidence exists that this native fluorescence or autofluorescence of bronchial tissues changes when they turn dysplastic and to carcinoma in situ. There is good agreement that the lesions display a decrease of autofluorescence in the green region of the spectrum under illumination with violet-light, and a relative increase in the red region of the spectrum is often reported. Imaging devices rely on this principle to detect early cancerous lesions in the bronchi. Based on a spectroscopic study, an industrial imaging prototype is developed to detect early cancerous lesions in collaboration with the firm Richard Wolf Endoskope GmbH, Germany. A preliminary clinical trial involving 20 patients with this spectrally optimized system shows that the autofluorescence can help to detect most lesions that would otherwise have remained invisible to an experienced endoscopist under white light illumination. A systematic off line analysis of the autofluorescence images pointed out that real-time decisional functions can be defined to reduce the number of false positive results. Using this method, a positive predictive value (PPV) of 75% is reached using autofluorescence only. Moreover, a PPV of 100% is obtained, when combining the white light (WL) mode and the autofluorescence (AF) mode, at the applied conditions. Furthermore, the sensitivity is estimated to be twice higher in the AF mode than in WL mode.

Endogenous microbial contamination of cultured autologous preparations in trials of cancer immunotherapy.

Standardization of the manufacturing and processing of cellular immunotherapy products is necessary to ensure patient safety, establish efficacy, and demonstrate potency. Recognition of the autologous donor as a likely source of microbial contamination of cellular immunotherapy products may improve patient care and reduce expense to the laboratory. Data on 243 immunotherapy products manufactured in the authors' institution between December 1997 and June 2001 were retrospectively reviewed. Also reviewed were the case reports of four patients whose autologous immunotherapy products were contaminated. Twenty-five (10%) of the 243 immunotherapy products processed were positive on one or more tests for microbial contamination. In six (24%) of the products, the source of microbial contamination was the autologous donor. In 17 of the remaining 19 products, test results were judged to be false-positive. The unique processing techniques and stringent controls involved in the manufacture of cellular immunotherapy products may result in changes in the sources of microbial contamination routinely encountered. The identification of the autologous donor as a potential source of the microbial contamination of the product may assist the clinician and the laboratory in troubleshooting products with positive results on microbial sterility testing. Also, the number of false-positive results in this study indicates that further research is needed to maximize the specificity of testing while maintaining the present high sensitivity.

Spécificité des techniques sérologiques de dépistage des anticorps anti-paludéens en immunofluorescence : étude comparative dans la pratique de qualification du don de sang

In France, no case of transfusion-transmitted malaria has been reported since 1994, reflecting the efficiency of the blood donations screening. However, an insufficiently specific serological assay leads to an unjustified loss of red cell units. Materials and methods: A retrospective study included 55 sera previously disqualified for the presence of malarial antibodies. We used different assays detecting these antibodies: Paludix™, Falciparum-Spot™ without or with the elimination of natural antiglobulins by two different methods. Results: Using Paludix™, 87% of sera were negative ( P < 0.001). With the sera treated for the elimination of natural antiglobulins by neutralisation or absorption, Falciparum-Spot TM gave respectively 87% and 94.5% of negative results. Conclusion: When using assays avoiding non-specific fluorescence, the number of false positive results significantly decreased. Because the number of blood donors travelling in at-risk countries is continuously rising, a sufficiently specific method to detect malarial antibodies should save a lot of red cell units.

Validade da citologia cervicovaginal na detecção de lesões pré-neoplásicas e neoplásicas de colo de útero

Introdução: O exame colpocitológico ou teste de Papanicolaou permite o diagnóstico precoce do câncer de colo do útero. O êxito no rastreamento desta doença dependerá, além de outros fatores, da acuidade diagnóstica do exame colpocitológico. Objetivos: Avaliar a acurácia diagnóstica da citologia cervicovaginal na detecção de lesões cervicais através da comparação citoistopatológica. Material e métodos: Avaliou-se o grau de concordância entre os exames citológico e histopatológico de 373 pacientes atendidas em um hospital universitário no período de 1990 a 1997 e foram calculados os indicadores: sensibilidade, especificidade, valores preditivos positivos e negativos e as taxas de falsos positivos e falsos negativos do exame. Resultados: A taxa bruta de concordância citoistopatológica foi de 65,1%. A sensibilidade do exame foi alta (96,0%), no entanto sua especificidade foi baixa (51,5%), significando a inclusão de muitos resultados falsos positivos. A taxa de falsos positivos foi de 48,4%, enquanto a de falsos negativos foi de 4%. Discussão: Apesar de a sensibilidade do teste ser alta, sua especificidade é baixa, significando que muitas mulheres seriam falsamente diagnosticadas como doentes, levando a um número elevado de resultados falsos positivos e custos desnecessários, além do potencial caráter iatrogênico que esta ação poderia assumir. No entanto, um exame altamente sensível é o teste de escolha para programas de rastreamento de câncer cervical na população feminina. Conclusões: Para garantir a acuidade diagnóstica do teste de Papanicolaou são essenciais atividades de controle de qualidade nos laboratórios, tanto do procedimento colpocitológico quanto do histopatológico, permitindo, assim, o êxito na detecção precoce e no tratamento das lesões cervicais.

Effect of acrylate chemistry on human health.

The prospective cohort study of 1992-1999 describes the effect of occupational exposure to chemical substances in the production of acrylic acid, acrylic acid esters and acrylate dispersions at the various workplaces of one chemical plant. Exposure to selected chemicals (acrylonitrile, n-butanol, butyl acrylate, ethyl acrylate, methyl acrylate, methyl methacrylate, toluene, and styrene) was determined by personal passive dosimetry (GC/MS method). The annual examinations included general health, by guided interview, a general medical examination, hematological and biochemical examinations, examination of the parameters of serum immunity and selected tumor markers, and spirometry. The authors also repeatedly performed cytogenetic analysis of human peripheral lymphocytes. The authors followed a group of 120 employees (60 exposed, 60 controls), mean age 40+/-8 years in both groups, with average period of exposure to chemicals (exposed group) 13+/-5 years. The measured concentrations of chemicals in the working atmosphere were generally low; maximum allowable concentrations (MAC) values or suggested limits of certain chemicals were occasionally exceeded (most frequently for butyl acrylate). The results of the examination of the workers over the 8 years have not revealed any marked differences between the exposed and control groups that could be attributable solely to the acrylate exposure. Exposure to chemical substances at the workplace was relatively low, the limits being exceeded only sporadically (each such case was investigated at the workplace), and the level of exposure continues to decrease gradually over the years. Considering the fact that the exposed individuals are expected to work for 23 additional years on average, we feel that long-term monitoring of selected health-related parameters, not including tumor markers, appears desirable. The examination of tumor markers has not contributed to the problem evaluation for a number of false-positive results.

Imaging Techniques for Evaluation of the Uterine Cavity and Endometrium in Premenopausal Patients Before Minimally Invasive Surgery

A literature review compared the diagnostic effectiveness and accuracy of transvaginal sonography (TVS) hysterosonographic examination (HSE), hysteroscopy (HY), and magnetic resonance imaging (MRI) in abnormalities of the uterine cavity and endometrium in premenopausal patients referred to surgery and women with abnormal uterine bleeding. The studies varied much in terms of patient selection, number of observers, blinding and experience of observers, and definition of abnormality criteria. The diagnostic effectiveness of the techniques reviewed varied: TVS only reached intermediate quality levels as a diagnostic tool for exclusion of uterine cavity abnormalities and no data support that MRI, TVS, HY, or HSE may exclude hyperplasia without concomitant endometrial sampling. HY and HSE were equally effective and apparently outperformed TVS, especially for identification of polyps. However, all techniques carried a significant number of false positive results. MRI does not satisfy current diagnostic demands for detection of endometrial abnormalities, but it is sufficiently accurate for submucous myoma (SM) evaluation. TVS, HSE, and HY carry much observer variation as opposed to MRI. In experienced hands TVS should be a first choice modality, but its precision and consistency fall short of current needs and it should therefore be supplemented by other techniques. HSE or HY performed by experienced clinicians should be used as supplements to TVS for exclusion of polyps. MRI can be recommended as the first choice modality for exact evaluation of SM uterine in-growth before advanced minimal invasive treatment of myomas. Clinicians should be aware that modern imaging techniques may yield highly idiosyncratic results when used by inexperienced staff, and efforts should be made to reduce such observer variation. Target Audience: Obstetricians & Gynecologists, Family Physicians Learning Objectives: After completion of this article, the reader will be able to describe the diagnostic strengths and weaknesses of the various diagnostic tests in the evaluation of abnormal uterine bleeding and to compare the accuracy of each test to the others.

Imaging techniques for evaluation of the uterine cavity and endometrium in premenopausal patients before minimally invasive surgery.

A literature review compared the diagnostic effectiveness and accuracy of transvaginal sonography (TVS) hysterosonographic examination (HSE), hysteroscopy (HY), and magnetic resonance imaging (MRI) in abnormalities of the uterine cavity and endometrium in premenopausal patients referred to surgery and women with abnormal uterine bleeding. The studies varied much in terms of patient selection, number of observers, blinding and experience of observers, and definition of abnormality criteria. The diagnostic effectiveness of the techniques reviewed varied: TVS only reached intermediate quality levels as a diagnostic tool for exclusion of uterine cavity abnormalities and no data support that MRI, TVS, HY, or HSE may exclude hyperplasia without concomitant endometrial sampling. HY and HSE were equally effective and apparently outperformed TVS, especially for identification of polyps. However, all techniques carried a significant number of false positive results. MRI does not satisfy current diagnostic demands for detection of endometrial abnormalities, but it is sufficiently accurate for submucous myoma (SM) evaluation. TVS, HSE, and HY carry much observer variation as opposed to MRI. In experienced hands TVS should be a first choice modality, but its precision and consistency fall short of current needs and it should therefore be supplemented by other techniques. HSE or HY performed by experienced clinicians should be used as supplements to TVS for exclusion of polyps. MRI can be recommended as the first choice modality for exact evaluation of SM uterine in-growth before advanced minimal invasive treatment of myomas. Clinicians should be aware that modern imaging techniques may yield highly idiosyncratic results when used by inexperienced staff, and efforts should be made to reduce such observer variation. Obstetricians & Gynecologists, Family Physicians. After completion of this article, the reader will be able to describe the diagnostic strengths and weaknesses of the various diagnostic tests in the evaluation of abnormal uterine bleeding and to compare the accuracy of each test to the others.

Nuclear medicine—the past predicts a bright future

Nuclear medicine, a field characterized by innovation since the time Becquerel and Curie defined the basic properties of radionuclides, continues to evolve in this seventh decade of the isotope scan. Nuclear medicine has always been ‘molecular’ [1] with early studies defining the avidity of the reticuloendothelial system, identifying the flux of ions across membranes and defining the size and turnover of important compartments, pools and spaces in the body. In contrast to anatomic imaging technology, nuclear medicine has made great strides in the development of new radiopharmaceuticals but has made little progress in imaging devices. Technical progress in technologies such as ultrasound and multidetector CT are remarkable. Harmonic imaging of native tissue (without added contrast) with ultrasound is an example of this. Harmonic imaging detects the second harmonic of the transmitted ultrasound pulse rather than imaging the fundamental frequency. Harmonic imaging provides better contrast than fundamental imaging as demonstrated in Fig. 1 of a complex renal cyst. Harmonic imaging has been applied to the heart to enhance delineation of lesions and perfusion. Heinle et al. [2] compared harmonic ECHO (with added echo contrast) to myocardial perfusion imaging with 99m Tc–sestamibi at rest and pharmacological stress. Images were obtained at baseline and during adenosine infusion in three apical views in 123 patients. Agreement between the two methods for detecting coronary disease in each of the three coronary territories was 81% for the left anterior descending artery, 76% for the right coronary artery and 72% for the left circumflex artery. Discrepancies between the two techniques were most notable in the circumflex territory, where fixed defects were observed in 33% by harmonic Doppler contrast ECHO but in only 14% by SPECT. This study concluded that harmonic Doppler contrast ECHO could reliably detect myocardial perfusion during pharmacological stress although there was a significantly higher number of false positive results in the circumflex territory.

How to detect current toxoplasma infection

This study seeks to identify the best way to detect current toxoplasma infection for district general hospital laboratories. One hundred ‘ordinary’ and 174 ‘difficult’ sera are categorised into either an ‘evidence’ or ‘no evidence’ group for current toxoplasma infection. Twelve test protocols are investigated using different combinations of one whole antibody latex test (Eiken Toxoreagent), one in-house specific IgG enzyme-linked immunosorbent assay (ELISA) and three specific IgM assays (Toxo-ISAGA, in-house BAM ELISA IgM and Toxonostika ELISA M). The Eiken latex and in-house IgG assays produced significantly fewer false-negative results than were obtained with the single IgM test or the IgG and IgM test protocols (P<0.05), but a greater number of false-positive results (102/274 and 115/274, respectively). Of the IgM assay test protocols, the three IgM assays in combination produced the least number of false-negative results (1/274). However, a significantly greater number of false-positive results were produced than with one or two IgM tests or an IgG and any IgM test in combination (P<0.001). We recommend testing with three IgM tests, or a whole antibody (Eiken) or IgG-specific assay, and that positive or clinically important negative samples be referred to a reference laboratory for confirmation.

Single photon emission tomography: Scintigraphy in the assessment of mandibular invasion by head and neck cancer

Background This study was performed to determine the effectiveness of single photon emission tomography (SPET) in contributing to the initial staging of patients with cancers of the head and neck because information about osseous infiltration of head and neck cancer is of major importance for staging and planning of treatment. Methods A retrospective analysis of 89 cases with probable tumor infiltration of the mandible by oral/pharyngeal cancer was undertaken by comparing the preoperative SPET as well as CT-scans, if available, with clinical and postoperative histological examinations. Results For SPET examinations, a sensitivity of 95% and a specificity of 48% were calculated. The positive predictive value was 65%, and the negative predictive value was 93%. CT scans showed a sensitivity of 75% and a specificity of 78%. The positive predictive value was 65%, and the negative predictive value was 93%. Conclusions SPET is a powerful method to detect infiltration of carcinomas into the head and neck skeleton. Its sensitivity is high and superior to CT scans, although the specificity is small due to a high number of false positive results. © 2001 John Wiley & Sons, Inc. Head Neck 23: 979–984, 2001.

Single photon emission tomography: scintigraphy in the assessment of mandibular invasion by head and neck cancer.

This study was performed to determine the effectiveness of single photon emission tomography (SPET) in contributing to the initial staging of patients with cancers of the head and neck because information about osseous infiltration of head and neck cancer is of major importance for staging and planning of treatment. A retrospective analysis of 89 cases with probable tumor infiltration of the mandible by oral/pharyngeal cancer was undertaken by comparing the preoperative SPET as well as CT-scans, if available, with clinical and postoperative histological examinations. For SPET examinations, a sensitivity of 95% and a specificity of 48% were calculated. The positive predictive value was 65%, and the negative predictive value was 93%. CT scans showed a sensitivity of 75% and a specificity of 78%. The positive predictive value was 65%, and the negative predictive value was 93%. SPET is a powerful method to detect infiltration of carcinomas into the head and neck skeleton. Its sensitivity is high and superior to CT scans, although the specificity is small due to a high number of false positive results.

Selective 'stencil'-aided pre-PCR cleavage of wild-type sequences as a novel approach to detection of mutant K-RAS.

The enriched PCR widely used for detection of mutant K-RAS in either tumor tissues or circulating DNA was modified so that abundant wild-type K-RAS alleles are cleaved prior to PCR. We took advantage of an AluI recognition site located immediately upstream of the K-RAS codon 12. The site was reconstituted upon DNA denaturation followed by annealing with a 'stencil', a 16-bp synthetic oligonucleotide complementary to the wild-type sequence. As opposed to normal K-RAS, the mutant allele forms, upon annealing with the stencil, a mismatch at the codon 12 which lies within the AluI enzyme binding site and partially inhibits its activity. The mismatch also lowers the melting temperature of the stencil-mutant K-RAS double helix as compared to stencil-wild-type duplex, so that only the latter is double stranded and selectively digested by AluI at elevated temperatures. The proposed method of stencil-aided mutation analysis (SAMA) based on selective pre-PCR elimination of wild-type sequences can be highly advantageous for detection of mutant K-RAS due to: (i) an enhanced sensitivity because of reduced competition with a great excess of normal K-RAS, and (ii) a decrease in a number of false-positive results from Taq polymerase errors. Application of SAMA for generalized detection of DNA mutations is discussed.