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PubMedDate not supplied

A clinic-based system for monitoring the quality of techniques for the diagnosis of gonorrhea.

The author describes a clinic-based system that monitors the quality of techniques for diagnosis of gonorrhea by continuous comparison of the results of gram-strained smears of urethral and cervical specimens with the corresponding cultures. Discrepant results are immediately reviewed. During a two-year period, the system detected two defects in culture incubation, visual defects in a laboratory technician, and three unexplained reductions in cultural sensitivity. The system motivates clinicians and laboratory technicians to maintain a high level of quality control. In 11,474 comparisons of urethral smear and culture results, there was agreement in 96.9% while the smear alone was positive in 1.6% and the culture alone in 1.5%. Review of positive-smear-negative-culture discrepancies indicated that most smears judged "false-positive" were so classified on the basis of false-negative cultures. In study of 1,331 women, the endocervical smeras identified 60% (273/455) of those with positive culture discrepancies. Many of these may have represented false-negative cultures.

Clinical Laboratory Techniques↗

PubMed1985

False-positive, follicular and irritant patch test reactions to metal salts.

853 hard metal workers were patch tested with nickel sulphate 5%, potassium dichromate 0.5% and cobalt chloride 1%, each in petrolatum. Non-allergic reactions appeared in 6.5% of the nickel tests, 13% of the chromium tests and 18.3% of the cobalt tests. Most of the individuals with positive, poral or pustular reactions were retested with serial dilutions of metal salts in pet. and in water. The accuracy of a positive initial nickel reaction was 83%, a chromium reaction 40% and a cobalt reaction 62%. The nonallergic reactions were partly reproducible and correlated with both the type of patch test material and with individual factors. Weak and moderately strong positive patch test reactions to metal salts may be irritant and should be checked with serial dilution tests or at least be retested. A reduction of the cobalt chloride concentration from 1% to 0.5% in the standard test material is discussed.

Chromates↗

PubMed1985

Selective detection and enumeration of fecal coliforms in water by potentiometric measurement of lipoic acid reduction.

Water samples of various origins were inoculated into a specific coliform-selective lactose broth provided with lipoic (thioctic) acid, and the time evolution of the redox potential of the cultures was monitored during incubation at 41 degrees C by use of gold versus reference electrodes. Positive potential-time responses, i.e., 100-mV potential shifts recorded within 20 h of inoculation, were related to the initial number of fecal coliforms in the broth determined by control enumeration techniques, and the organisms responsible were isolated and identified by conventional procedures. A total of 30 samples of wastewater, 38 of surface water, 553 of groundwater, and 110 of drinking water were tested successively. A total of 240 natural water samples, including 172 groundwater samples, and 1 drinking water sample were found to be positive in the potentiometric test. The majority (i.e., 92.5%) of the relevant potentiometric detection times were shorter than 15 h, and 96% of these could be attributed to Escherichia coli. Fifteen hours corresponded to the limit for detecting 1 E. coli cell per 100 ml of water. About 78% of the potentiometric responses occurring after 15 h were induced by fecal coliforms other than E. coli (Enterobacter cloacae, Klebsiella pneumoniae, and Citrobacter freundii). Calibration curves relating detection times shorter than 15 h to fecal coliform (i.e., E. coli) concentrations were constructed for the natural water samples tested. There were minor variations in the average growth rate of the organisms in the relation to the contamination level of the water tested. The number of false-positive samples in the potentiometric test was equivalent to that of false-negative samples (groundwater or drinking water).

Drinking↗

PubMed1996

New laboratory guidelines for serologic diagnosis of Lyme disease: evaluation of the two-test protocol.

Recent guidelines established by the Association of State and Territorial Public Health Laboratory Directors (ASTPHLD) and the U.S. Centers for Disease Control and Prevention (CDC) recommend the use of a two-test protocol for the serologic diagnosis of Lyme disease (LD). The two-test protocol relies on a sensitive screening test, which is followed by specific immunoglobulin M (IgM) and/or IgG immunoblotting (IB), depending on the date of disease onset, of all samples with equivocal and positive screening test results. We evaluated a commercially available IgM-IgG enzyme-linked immunosorbent assay (ELISA) and separate IB tests for IgM and IgG antibodies to Borrelia burgdorferi as candidate assays for the two-test protocol. Serum samples obtained from healthy controls (n = 29), from patients with diagnoses or laboratory findings associated with serologic cross-reactivity to LD (n = 24), and from patients with well-documented early- and late-stage LD provided by the CDC and the College of American Pathologists (n = 53) were examined to determine each assay's individual sensitivity and specificity. No false-positive results were detected among the healthy controls by either ELISA or IB, whereas four false-positive ELISA results were recorded within the cross-reactive group. None of these sera, however, were positive for either IgM or IgG reactivity according to IB band criteria. With regard to the patients with LD, we determined the sensitivity and specificity of the ELISA to be 96 and 100%, respectively, compared with the reference data provided for these specimens. When we compared our IB results with data from CDC, the assay sensitivity and specificity were 80 and 96.2%, respectively, for IgM and 81.8 and 95.8%, respectively, for IgG. Pursuant to this evaluation we assessed the suitability of the two-test protocol by performing a retrospective analysis using clinical history to define samples as positive or negative for LD. We determined clinical sensitivity and specificity for all study subjects (n = 112) to be 50 and 100%, respectively. A reduction in the clinical sensitivity of the two-test protocol was associated with a lack of antibody response or seroconversion in LD patients treated with antibiotics. We conclude that the CDC-ASTPHLD guidelines provide useful criteria for test performance and interpretation aimed at standardizing the serologic diagnosis of LD.

Borrelia burgdorferi Group↗

PubMed1997

Rapid identification of Candida species in blood cultures by a clinically useful PCR method.

Widespread use of fluconazole for the prophylaxis and treatment of candidiasis has led to a reduction in the number of cases of candidemia caused by Candida albicans but has also resulted in the emergence of candidemias caused by innately fluconazole-resistant, non-C. albicans Candida species. Given the fulminant and rapidly fatal outcome of acute disseminated candidiasis, rapid identification of newly emerging Candida species in blood culture is critical for the implementation of appropriately targeted antifungal drug therapy. Therefore, we used a PCR-based assay to rapidly identify Candida species from positive blood culture bottles. This assay used fungus-specific, universal primers for DNA amplification and species-specific probes to identify C. albicans, C. krusei, C. parapsilosis, C. tropicalis, or C. glabrata amplicons. It also used a simpler and more rapid (1.5-h) sample preparation technique than those described previously and used detergent, heat, and mechanical breakage to recover Candida species DNA from blood cultures. A simple and rapid (3.5-h) enzyme immunosorbent assay (EIA)-based format was then used for amplicon detection. One hundred fifty blood culture bottles, including 73 positive blood culture bottle sets (aerobic and anaerobic) from 31 patients with candidemia, were tested. The combined PCR and EIA methods (PCR-EIA) correctly identified all Candida species in 73 blood culture bottle sets, including bottles containing bacteria coisolated with yeasts and 3 cultures of samples from patients with mixed candidemias originally identified as single-species infections by routine phenotypic identification methods. Species identification time was reduced from a mean of 3.5 days by routine phenotypic methods to 7 h by the PCR-EIA method. No false-positive results were obtained for patients with bacteremias (n = 18), artificially produced non-Candida fungemias (n = 3), or bottles with no growth (n = 20). Analytical sensitivity was 1 cell per 2-microl sample. This method is simpler and more rapid than previously described molecular identification methods, can identify all five of the most medically important Candida species, and has the potential to be automated for use in the clinical microbiology laboratory.

Candida↗

PubMed1997

Comparison of screening methods for detection of extended-spectrum beta-lactamases and their prevalence among blood isolates of Escherichia coli and Klebsiella spp. in a Belgian teaching hospital.

Using a set of 33 well-defined extended-spectrum beta-lactamase (ESBL)-producing strains of Escherichia coli and Klebsiella pneumoniae, we compared three screening methods for ESBL detection: (i) a double-disk synergy test, (ii) a three-dimensional test (both the double-disk synergy test and the three-dimensional test were performed with ceftriaxone, ceftazidime, aztreonam, and cefepime), and (iii) the Etest ESBL screen (AB Biodisk, Solna, Sweden), based on the recognition of a reduction in the ceftazidime MIC in the presence of clavulanic acid. In the double-disk test, all four indicator antibiotics scored equally and 31 of the 33 reference strains were recognized. In the three-dimensional test, ceftriaxone was the only satisfactory indicator and 30 ESBL-positive strains were detected by this antibiotic. Both systems produced two false-positive results with cefepime. With the Etest ESBL screen, 15 of 16 TEM-related and 11 of 16 SHV-related ESBL-producing strains scored positive. In 10 cases the clavulanic acid on one end of the strip interfered with the MIC determination for ceftazidime, which was read on the opposite end. This MIC had to be determined with an extra ceftazidime-only strip. No false-positive results were noted. Eighty-six blood isolates of E. coli and Klebsiella species were screened for ESBL expression by the double-disk and three-dimensional tests, both with ceftriaxone. Six strains with suspicious antibiogram phenotypes also gave positive results by the double-disk test. One E. coli strain remained undetected by the three-dimensional test. Identification of the enzymes suspected of being ESBLs by isoelectric focusing (all strains) and DNA sequencing (1 strain) confirmed the screening test results except for one Klebsiella oxytoca strain, which proved to be a hyperproducer of its chromosomal enzyme and which also had a negative Etest score. The five true ESBL producers were all confirmed by the Etest ESBL screen. Pulsed-field gel electrophoresis proved that the E. coli strains were unrelated, but that two of the three K. pneumoniae strains were closely related.

Aztreonam↗

PubMed1981

Trypsinised human O erythrocytes in the detection of rubella-specific IgM by sera fractionation on sucrose density gradient and absorption with staphylococcal protein A.

Detection of rubella virus-specific IgM employing trypsin-treated human group O erythrocytes was evaluated using the method of sera fractionation on sucrose density gradients (SDG) and that of sera absorption with staphylococcal protein A. The former method proved to be highly specific and sensitive in confirming or excluding rubella by demonstration of specific IgM. In contrast, the latter method provided comparable results in only 71.43% of specimens tested by both methods while false-positive or -negative IgM results were obtained in the remaining 28.57% of specimens. In view of these results, therefore, it is recommended that all those specimens found positive for specific IgM by the protein A method must be confirmed by another procedure, possibly that of specific IgM reduction with 2-mercaptoethanol.

ABO Blood-Group System↗

PubMed1979

Results of ophthalmosonometry and supraorbital photoplethysmography in evaluating carotid arterial stenoses.

A total of 263 patients underwent either periorbital ophthalmosonometry (Doppler ultrasound) or a combination of ophthalmosonometry and supraorbital photoplethysmography (SOPPG) to determine the presence or absence of a hemodynamically significant lesion (occlusion or greater than 50% reduction in lumen diameter). There were 522 vessels studied with Doppler ultrasound and 272 vessels with a combination of the two modalities. The tests were correlated with carotid arteriography to determine their accuracy. Overall accuracy of the Doppler ultrasound examination was 95%. A normal Doppler ultrasound examination was obtained in 367 vessels and arteriography confirmed the absence of a hemodynamically significant lesion in 93%. Of 155 vessels with an abnormal Doppler examination, 149 (96%) were proven on arteriography to have hemodynamically significant lesions. The Doppler ultrasound examination had a high incidence of false-negative examinations (26%) in those vessels that were between 50% and 99% stenotic, however. The SOPPG alone had a high incidence of false-positive examinations (19%); however, in 155 vessels with a normal SOPPG, 98% were proved to have no hemodynamically significant lesion. The combination of Doppler ultrasound and SOPPG was highly accurate in identifying patients with hemodynamically significant lesions. Of 97 vessels with such lesions, 96 (99%) had either an abnormal Doppler ultrasound examination or an abnormal SOPPG. In 240 vessels with concordant results of the Doppler ultrasound and SOPPG, the presence or absence of a hemodynamically significant lesion was accurately predicted in 235 (98%).

Arterial Occlusive Diseases↗

PubMed1975

Evaluation of the usefulness of nitroblue tetrazolium reduction test (NBT test) for detection of bacterial infection in cancer patients.

False positivity of the NBT test in neoplastic diseases described by some authose may be a limitation in the use of the test for infection screening in cancer patients. For explaining this problem studies were performed in a group of 30 patients with various untreated malignancies without infection and in 20 of these with bacteriologically confirmed infections. The obtained results were compared with those in groups of normal persons without and with infection. Slightly modified Park method for spontaneous and stimulated NBT test was used. No significant differences between patients of both groups without infection were found. The increase in positivity of the test in presence of infection was lower in cancer patients than in normal subjects, however, it was non-significant. We did not observe any false-positive results in our patients, though one case of false negativity in myeloma is described separately. It is concluded that neoplastic disease does not cause, as such positivity of the NBT test. Because of similar response of NBT test to presence of infection, the NBT test is recognized as very useful for infection screening in cancer patients similarly as in normal persons.

Adult↗

PubMed1992

[Echographic measuring of the length of the fetal femur in the screening for Down syndrome].

OBJECTIVE: Our purpose was to assess the value of sonographic measurement of fetal femur length in the second trimester, as a screening tool for Down's syndrome. STUDY DESIGN: We evaluated a consecutive series of fetuses scanned by a single sonologist at the time of amniocentesis between 15 and 19 weeks. The study group consisted of fetuses with Down's syndrome (N = 16); the control group comprised normal fetuses (N = 1163). A linear regression model of the normal femur length based on biparietal diameter (BPD), was established for our population; the ratios of measured to expected femur length for a viven BPD (FL M/E) were calculated in the two groups. To test statistical significance of observed differences between case and control population, unpaired t test was used. The ability of specific FL M/E cut-off values to discriminate between Down syndrome and normal fetuses was assessed by Fisher's exact test. RESULTS: The mean ratio of measured to expected femur length was significantly lower in the Down syndrome as compared with control population (0.9473, DS 0.0795) versus 1.0, DS 0.0745) (p < 0.0045). The ratio of 0.91 or less predicted Down's syndrome with a sensitivity of 43.7% and a false-positive rate of 8.6%. For women with risk of one in 250 and one in 1000 of having an affected fetus based on maternal age, a shortened femur yielded positive predictive values of one in 26 and one in 105, respectively. CONCLUSION: These results suggest that the sonographic measurement of fetal femur length for the screening of Down syndrome in the low-risk population is hindered by a high false positive rate (about 9%). It follows that the percentage of women requiring an amniocentesis would increase to un unacceptably high level. The utilization of this biometric marker may be helpful, in our opinion, for identifying fetuses at risk for Down syndrome in women between 35 and 38 years of age. These women in fact, are not offered amniocentesis for the prenatal diagnosis of Down's syndrome in the majority of italian institutions. The sonographic measurement of fetal femur length could detect about 45% of fetal Down's syndrome, offering an amniocentesis to 9% of women, with a consequent reduction of the cost required. Only a prospective study can evaluate the efficacy of this method to predict Down syndrome in such group of women.

Adult↗

PubMed1990

[Measuring temporal transmission properties of the visual system. Diagnostic value in suspected optic nerve neuritis].

The temporal transfer properties of the visual system were examined in 157 patients with suspected acute optic neuritis or papillitis. Foveal modulation sensitivity was measured as a function of temporal frequency (DeLange curve) both for the affected eye and the unaffected fellow eye. Patients with acute optic neuritis typically show a reduction of foveal modulation sensitivity at all temporal frequencies, high temporal frequencies above 10 cps being more markedly affected. The aim of the present study was to determine the sensitivity and specificity of this simple psychophysical procedure for the diagnosis of neuritis or papillitis in a reasonably large population of patients with suspected acute optic neuritis or papillitis. In each case the final diagnosis was confirmed by evaluation of the clinical signs and symptoms, the course of disease and additional diagnostic procedures, e.g. electrophysiology, perimetry, neurological examination including CT, lumbal puncture and examination of cerebrospinal fluid. In 136 cases (86.6%) the result read from the DeLange curve was correctly positive or negative. In 17 cases (10.8%) the DeLange curve indicated a false-positive and in 4 cases (2.6%), a false-negative result.

Flicker Fusion↗

PubMed1990

Strategies for screening blood for human immunodeficiency virus antibody. Use of a decision support system.

A decision analytic model was used to examine alternative strategies to screen donated blood for human immunodeficiency virus (HIV) using data from the literature and from 1987 blood-screening programs in areas with high and low prevalence of HIV. Sensitivity analyses incorporated uncertainties about HIV infection and test performance. Current screening strategies are estimated to allow 20.5 infected units per million donated units to be transfused at a cost of $16,850 per HIV-positive unit detected in high-prevalence areas and 4.7 infected units per million donated units to be transfused at a cost of $32,275 per HIV-positive unit detected in low prevalence areas, with nine false-positive notifications of uninfected patients per million units screened and 14.9 discarded, noninfected units per HIV-positive unit in low-prevalence areas. Testing donated blood for HIV can be improved by individualizing screening strategies for areas with different prevalences of HIV. Efforts to further reduce transfusion-associated HIV should focus on improved test performance in early stages of infection, reduction of unnecessary transfusions, donor recruitment in lower-risk groups, and public health measures to reduce HIV infection among the general population.

Blood Donors↗

PubMed1979

Scintigraphic, electrocardiographic, and enzymatic diagnosis of perioperative myocardial infarction in patients undergoing myocardial revascularization.

To assess the incidence of perioperative myocardial infarction, 214 consecutive patients were evaluated 1-5 days after coronary bypass surgery, using Tc-99m pyrophosphate (TcPPi) myocardial imaging, serial electrocardiograms (ECG), and enzyme levels (SGOT, LDH, CPK). On the basis of the clinical course and scintigraphic, enzymatic, and ECG changes, the diagnosis of perioperative infarction was definite in 17 of 241 cases (7.9%) and probable in six of 241 (2.8%). In all of these 23 patients, TcPPi scans were abnormal; one additional patient had a false-positive scintigram. Only 13 of the 23 had ECG evidence of infarction, but there were no false positives. We set the threshold for abnormality of enzyme changes quite high, owing to experience in more than 900 postoperative patients (SGOT greater than 200, LDH greater than 500, CPK greater than 500 on the same day). Using these criteria, 22 of the 23 infarct patients had abnormal enzymes, and six others were falsely positive. These results indicate a relatively low sensitivity for the ECG in diagnosing perioperative infarction, but the lack of false positives suggests high specificity. The sensitivity and specificity of the enzymes and the TcPPi image were both excellent and quite similar; the main difference was a reduction of certainty of infarction with the enzyme criteria, caused by the six patients whose enzyme values were falsely positive. Considering its sensitivity, specificity, and ability to locate and to a certain extent quantitate necrosis. TcPPi imaging is probably the most valuable means of diagnosing perioperative myocardial infarction.

Clinical Enzyme Tests↗

PubMed1982

[Comparison between the loading test and selective coronary angiography in stenocardia].

The results from the step-wise loading test to 75 per cent of the age maximum pulse rate, performed by veloergemeter or thread-mill in 52 patients (9 females and 43 males, II of them with atypical chest pain and II with stenocardia) were juxtaposed to the data from the selective coronarography. It was established that the a reduction of ST segment greater than I mm and the appearance of precordial pain, degree III by the five-grade scale, have almost identical specificity (70% and 64% resp) and a predicting value of the positive result (68% and 70% resp) and a slightly higher sensitivity to pain (68% and 84% resp) in the detection of coronary stenosis greater than 50 per cent of the diameter of a main coronary vessel. The combination of the signs precordial pain degree III and/or ST reduction greater than I mm and/or elevation of ST greater than 2 mm, with same predicting value (67%) maintained, but with a considerably enhanced sensitivity (96%) proved to be most adequate as a criterion of the positive test. A reduction of ST segment greater than 22 mm is characterized by decrease of sensitivity (40%) but with a considerable increase of specificity (96%). The positivation of that sign suggests the presence mainly of a multibranch disease. The patients with coronary stenosis greater than 50%, rarely reach a physical capacity over 100 wt (7 x oxygen consumption) and a product of the maximum reached pulse rate and systolic blood pressure over 20 000 as compared with those without stenosis, but no difference among the patients with one-branch and multi-branch disease was established. The electrocardiographic changes in the patients with a true positive test with loading is more often retained after 4th minute of the rehabilitation phase as compared with those of the patients with false-positive test. Evidence exists to admit that the predicting value of the positive test is poorer in the patients with atypical pains and females.

Adult↗

PubMed1994

[Chlamydia infection and CIN].

Under the hypothesis that a chronic infection of the cervical epithelium leads to a reduction in the immunological response, a study was carried out to compare the possible relationship between chlamydial infection and CIN. 187 patients were referred to hospital for histological confirmation because of a cytological smear result (PAP III and IV a). Smears were examined for chlamydia antigens using immunofluorescence. The presence of antichlamydial IgA and IgG was performed in serum using an indirect immunohistochemical method. These tests confirmed CIN in 163 patients. The infection rate with chlamydia was 52.9%, 15.9% of CIN patients showing an active chlamydial infection. Inclusion of topographical findings, together with differential evaluation of colposcopic results, with reference to the transformation zone, allow for a separation between inflammation and dysplasia. The confirmation of a positive chlamydial infection and colposcopic criteria may help to reduce the high percentage of false-positive findings in the problem-group of PAP III patients. It appears that reservations about smear controls after doxycyclin therapy--including treatment of sexual partners--is justified.

Antibodies, Bacterial↗

PubMed1993

Role of transesophageal echocardiography in dissection of the aorta and evaluation of degenerative aortic disease.

The combination of different ultrasound techniques such as transthoracic, suprasternal, subcostal, and TEE has a high sensitivity and specificity in the diagnosis of aortic dissection. Limitations of this combined ultrasound technique are related to the visualization of the ascending part of the aortic arch, which, because of the interposition of the trachea, cannot be visualized completely. The beginning or end of a dissection in this part of the aorta may be misinterpreted. However, false-negative results are rare. False-positive results due to artifacts resulting from reverberations in an ectatic ascending aorta must be taken into account. The most important diagnostic goals in acute or chronic aortic dissection are (1) confirmation of the diagnosis by visualization of the intimal membrane; (2) the differentiation of true and false lumen, depending on visualization of spontaneous echocardiographic contrast, thrombus formation, slow or reduced reversed flow, systolic diameter reduction, and signs of entry jet into the false lumen; (3) detection of intimal tear, demonstrating communication by 2-D or color Doppler echocardiography; (4) determination of the extent of dissection with classification according to DeBakey types I, II, and III, or Stanford types A and B, with differentiation between communicating or noncommunicating dissection and antegrade or retrograde dissection limited to the descending aorta or expanding into the ascending aorta; (5) detection of wall motion abnormalities as a sign of preexisting coronary artery disease or myocardial ischemia due to ostium occlusion by an intimal flap, coronary artery rupture, or collapse of the true lumen during diastole; (6) detection and grading of aortic insufficiency; (7) detection of side branch involvement by suprasternal, subcostal, and abdominal sonography (which will provide information about the choice of the site for cannulation or catheterization of the femoral artery); and (8) detection of pericardial or pleural effusion and mediastinal hematoma as signs of an emergency situation (i.e., suspending rupture). Based on ultrasound diagnostic information, operation can be performed in all acute situations in patients with type A dissection without further investigation. The ability to act decisively in this setting is particularly important in patients with signs suggesting a dire prognosis (i.e., pericardial or pleural effusion or mediastinal hematoma). For follow-up studies, the combination of echocardiography with MR tomography is recommended. With TEE, entry tears can be detected with a higher sensitivity than with MR tomography. This capability may be important for the patient's prognosis. MR tomography, on the other hand, has a better spatial resolution showing the entire aorta, particularly the ascending aortic arch.

Aged↗

PubMed1995

The signal-averaged ECG becomes late potential-positive at low noise levels in healthy subjects.

Late potentials are detected at various noise levels in clinical studies. The aim of this study was to quantify the effect of residual noise level on the signal-averaged ECG. Thirty minutes of raw ECG (lead -X, +X, -Y, +Y, -Z, and +Z) from 10 healthy volunteers were digitized and stored on optical discs. Each ECG was analysed by four signal averaging procedures to noise level 0.1, 0.2, 0.3, and 0.4 microV, respectively. At each noise level, time domain analysis of the filtered vector (40-250 Hz) included determination of the QRS duration (QRS), late potential duration (LPD, duration of terminal signals below 40 microV), and root-mean-square voltage of the terminal 40 ms of the filtered QRS (RMS40). On average, the measured signal-averaged QRS duration was prolonged by 7.0 ms (range 0.9-12.5 ms) per 0.1 microV reduction in noise level. Late potential duration increased by 5.9 ms (range 0.2-10.0 ms) per 0.1 microV reduction in noise level, and RMS40 was reduced by 9.1 microV (range 0.5-20.2 microV) per 0.1 microV reduction in noise level. At noise level 0.4 microV, 0.3 microV, 0.2 microV, and 0.1 microV the number of late potential-positive subjects were 1, 2, 4, and 6, respectively. Late potential parameters are significantly dependent on noise level after signal averaging. Using conventional criteria for late potentials, healthy subjects become false-positive at low noise levels. Establishment of standards for noise level is recommended.

Adult↗

PubMed1996

Spatial frequency dependent observer bias in the measurement of contrast sensitivity.

A previously reported spatial frequency dependent change in contrast sensitivity, apparently produced by the psychometric method used, was investigated in two experiments. A relative reduction in the measured contrast sensitivity function at higher spatial frequencies was found when using a simple single staircase procedure, even when all external characteristics of the presentation procedure were the same as a more sophisticated and more reliable psychometric method (adaptive probit estimation). The estimated false-negative response rate was found to be greater for the single staircase procedure and was greater at a higher spatial frequency, while the false-positive response rate to interleaved blank presentations did not vary between spatial frequencies and between the two psychometric methods. This supports the suggestion that some subjects, given a knowledge of the psychometric method controlling grating contrast, may adopt different decision criteria at different spatial frequencies.

Adult↗