Interactive Threshold Research Articles

Comparison of three optical coherence tomography scanning areas for detection of glaucomatous damage

Several cross-sectional studies have demonstrated the capability of optical coherence tomography (OCT) to detect glaucomatous changes. OCT enables posterior pole scanning of three regions: macula, peripapillary, and optic nerve head (ONH). This study compared the ability of each region to detect glaucomatous damage. Retrospective observational cross-sectional study. The study included 37 normal (37 subjects) and 37 glaucomatous eyes (26 subjects) that had comprehensive ocular examination, reliable and reproducible Swedish interactive thresholding algorithm standard 24-2 perimetry, and Stratus OCT scanning of macula, peripapillary, and ONH regions on the same visit. Optical nerve head (ONH) appearance did not form part of the inclusion criteria. The main outcome measure, was area under receiver operating characteristic curves (AROCs) that was calculated for each scanning region for distinguishing between normal and glaucomatous eyes. The highest AROCs for distinguishing between groups were for ONH parameters (rim area = 0.97, horizontal integrated rim width = 0.96, vertical integrated rim area = 0.95) and peripapillary nerve fiber layer (NFL) thickness (0.94) followed by macular volume and thickness (both 0.80). A statistically significant difference existed in ONH and NFL AROCs when compared with macular AROCs (P < or = .007, for both) OCT ONH and NFL parameters provided similar discrimination capabilities between healthy eyes and those of glaucoma patients and superior discrimination capabilities when compared with macular parameters.

Post-Examination Processing in the SITA Standard Algorithm Compromises the Advantage of a Faster Patient Testing Time

Purpose To investigate the time taken for Swedish Interactive Threshold Algorithm (SITA)-standard (SITA-s) data management from first stimulus to printed results.

Analysis of an interaction threshold in a mixture of drugs and/or chemicals

Increasingly, humans are exposed to drug/chemical mixtures. These exposures can result from therapeutic interventions or environmental sources. Of interest is the interaction that may occur among the components of these mixtures. Since interaction can be dose-dependent, it is important to determine exposure levels to either exploit the benefits of the interaction in a therapeutic application or to avoid the effect of the interaction in the case of an environmental risk assessment. We propose generalized linear models that permit the estimation of interaction threshold boundaries. The methods developed are applied to the combination of ethanol and chloral hydrate.

Quantitative Assessment of Percent Breast Density: Analog versus Digital Acquisition

Breast density is a moderate risk factor for breast cancer based on quantitative measurement of percent breast density from film-screen mammograms. In this study, percent breast density was determined using computer-assisted interactive thresholding software from sixty consecutive mammograms of women undergoing digital screening mammography with a prior film-screen mammogram obtained within the last two years. Observations were made regarding discrepancies in density readings. Percent breast density was significantly lower for digital mammograms (mean 32.2%) compared to analog mammograms (mean 40.3%) (p<0.0001). This was not significant for women with less than 20% breast density (range +0.3 to -2.7%), but larger differences were seen with increasing density (12.5-14.9% lower for >50% density). Differences in density readings between analog and digital mammography were largely observed to be due to better recognition of the skin line on digital mammograms resulting in inclusion of more subcutaneous fat. Difficulties with appropriate recognition of subcutaneous breast tissue and fatty tissue near the chest wall were present for both analog and digital mammography. In conclusion, percent breast density is significantly lower when the mammogram is acquired in digital format compared to film-screen, largely due to better recognition of the skin line with resultant inclusion of more subcutaneous fat. Breast cancer risk predictions based on computerized assessment of breast density may be underestimated when applied to digital mammography.

Chemical mixture toxicology: from descriptive to mechanistic, and going on to in silico toxicology

Because of the pioneering vision of certain leaders in the biomedical field, the last two decades witnessed rapid advances in the area of chemical mixture toxicology. Earlier studies utilized conventional toxicology protocol and methods, and they were mainly descriptive in nature. Two good examples might be the parallel series of studies conducted by the U.S. National Toxicology Program and TNO in The Netherlands, respectively. As a natural course of progression, more and more sophistication was incorporated into the toxicology studies of chemical mixtures. Thus, at least the following seven areas of scientific achievements in chemical mixture toxicology are evident in the literature: (a) the application of better and more robust statistical methods; (b) the exploration and incorporation of mechanistic bases for toxicological interactions; (c) the application of physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) modeling; (d) the studies on more complex chemical mixtures; (e) the use of science-based risk assessment approaches; (f) the utilization of functional genomics; and (g) the application of technology. Examples are given for the discussion of each of these areas. Two important concepts emerged from these studies and they are: (1) dose-dependent toxicologic interactions; and (2) “interaction thresholds”. Looking into the future, one of the most challenging areas in chemical mixture research is finding the answer to the question “when one tries to characterize the health effects of chemical mixtures, how does one deal with the infinite number of combination of chemicals, and other possible stressors?” Undoubtedly, there will be many answers from different groups of researchers. Our answer, however, is first to focus on the finite (biological processes) rather than the infinite (combinations of chemical mixtures and multiple stressors). The idea is that once we know a normal biological process(es), all stimuli and insults from external stressors are merely perturbations of the normal biological process(es). The next step is to “capture” the biological process(es) by integrating the recent advances in computational technology and modern biology. Here, the computer-assisted Reaction Network Modeling, linked with PBPK modeling, offers a ray of hope to dealing with the complex biological systems.

Assessment of protein spot components applying correspondence analysis for peptide mass fingerprint data

Proteins separated by two-dimensional gel electrophoresis (2-DE) may be distributed over several spots. Otherwise, one spot may contain more than one component. The same protein occurring in several spots supposedly represents differently modified protein species that might be of biological relevance. Identification of spots with peptide mass fingerprinting and database searching leads only to the detection of the major spot components. If a spot also contains additional minor protein components, quantitation of spots with protein staining techniques or antibody detection becomes misleading. In order to find spots containing minor components we applied correspondence analysis, a multivariate data exploration method, to peptide mass fingerprint data. Correspondence analysis using peak lists revealed groups of spots containing the same protein with their characteristic mass-to-charge ratio (m/z) values. In order to detect different protein spot components an interactive threshold setting and removal of m/z values with subsequent recalculation of the correspondence analysis using our software tool CorrAn are performed. The usefulness of this methodical approach was shown by a data set of peptide mass fingerprints of 284 spots of Helicobacter pylori 26695 separated by 2-DE.

Swedish Interactive Thresholding Algorithm Fast for following visual fields in prepubertal idiopathic intracranial hypertension

Objective To examine the reliability of Swedish Interactive Thresholding Algorithm Fast (SITA Fast) visual fields (VFs) in prepubertal idiopathic intracranial hypertension (IIH) and to compare whether age, gender, or severity of visual outcome influenced the reliability of VF tests. Design Prospective, longitudinal cohort study. Participants Twenty-six prepubertal children (<11 years of age; mean age, 7.2 years) with IIH. Testing Children were prospectively followed up using a child-oriented program, which included SITA Fast VF tests. Age, gender, and severity of visual outcome were correlated with reliability of performance on SITA Fast VFs using a 1-way analysis of variance, point-biserial correlation, and the chi-square test for independence of observation. Main outcome measures Statistical analyses results that correlated the reliability of SITA Fast VFs with age, gender, and visual outcome. Results Three children were treated at the age of 2 years and were unable to perform automated VF tests. Four children were treated at the age of 4 years, 3 of whom were able to perform repeatable reliable SITA Fast VF tests. Forty-two eyes of 21 children had an average SITA Fast test time of 4.5 minutes for each eye, compared with 8 minutes using the Fastpac strategy. Age was not associated with reliability scores of SITA Fast tests ( F = 0.971, not significant [ns]). Gender did not influence the reliability of SITA Fast VF tests (chi-square (1) = 0.669, ns), nor did severity of visual outcome (chi-square (2) = 3.348, ns). Visual deficits were observed in 55% of patients at presentation and in 27% of patients after resolution of papilledema. Conclusions The SITA Fast VF tests can be performed from age 4 years and offer a reliable method for shortening test time. A child-oriented follow-up program, which entails a shortened testing time, may improve outcome in prepubertal IIH.

Effect of cataract extraction on frequency doubling technology perimetry

Purpose To investigate the effect of cataract extraction on the results of frequency doubling technology (FDT) perimetry in healthy subjects. Design Single-center, prospective, case series. Methods We performed FDT threshold C20-1 and Humphrey Swedish Interactive Threshold Algorithm (SITA)–fast programs within 1 month before and 2 months after phacoemulsification in 52 consecutive nonglaucomatous patients. Global indexes, including mean deviation and mean sensitivity of FDT and mean deviation of SITA-fast, were compared before and after cataract surgery. Mean sensitivity of FDT at various visual field grid locations was also compared before and after cataract surgery, as were localized indexes of both FDT and SITA-fast. Results Mean deviation improved after cataract surgery in both FDT threshold (from −7.09 dB–−2.16 dB) and SITA-fast program (from −6.14 dB–−2.87 dB). The mean sensitivity at 17 grid locations when the FDT threshold program was used significantly improved postoperatively (from 19.6 dB–26.13 dB). Postoperative improvements in mean sensitivity were also significant at all different sectors, but there were no significant differences of improvement among them. Postoperative pattern standard deviations from FDT and SITA-fast did not differ significantly from their preoperative values. Conclusions Cataracts induced a generalized reduction of sensitivity in FDT threshold as well as SITA-fast perimetry. Cataract removal did not change pattern standard deviation of FDT or of SITA-fast. Caution should be taken when interpreting the results of FDT perimetry in the eyes with cataract.

Space–time foam and cosmic-ray interactions

It has been proposed that propagation of cosmic-rays at extreme-energy may be sensitive to Lorentz-violating metric fluctuations (“foam”). We investigate the changes in interaction thresholds for cosmic-rays and gamma-rays interacting on the CMB and IR backgrounds, for a class of stochastic models of space–time foam. The strength of the foam is characterized by the factor (E/MP)a, where a is a phenomenological suppression parameter. We find that there exists a critical value of a (dependent on the particular reaction: acrit∼3 for cosmic-rays, ∼1 for gamma-rays), below which the threshold energy can only be lowered, and above which the threshold energy may be raised, but at most by a factor of two. Thus, it does not appear possible in this class of models to extend cosmic-ray spectra significantly beyond their classical absorption energies. However, the lower thresholds resulting from foam may have signatures in the cosmic-ray spectrum. In the context of this foam model, we find that cosmic-ray energies cannot exceed the fundamental Planck scale, and so set a lower bound of 108 TeV for the scale of gravity. We also find that suppression of p→pπ0 and γ→e−e+ “decays” favors values a≳acrit. Finally, we comment on the apparent non-conservation of particle energy–momentum, and speculate on its re-emergence as dark energy in the foamy vacuum.

Two interacting particles in an external potential

The solution to the Schrodinger equation is analyzed for two particles interacting in an external potential. It is assumed that a shallow level exists for one of the particles in the external potential. It is shown that repulsion leads to a transfer of one of the particles to the continuous spectrum after attainment of the interaction threshold. For the shallow level, the threshold value of the interaction is independent of level depth.

A mammographic image analysis method to detect and measure changes in breast density

We present an image analysis method that can detect and measure breast density from digitised mammograms. We present initial results on applying our method to characterise breast changes, in particular, changes due to Hormone Replacement Therapy (HRT). It has been established that long-term use of certain hormone replacement therapies can increase the risk of breast cancer, a fact that encourages the notion that objective measures of tissue density can be an important development in breast cancer image analysis. A set of 59 temporal pairs of mammograms of patients undergoing HRT (two images per patient) were used. The clinician’s assessment of density changes constituted the ground truth for evaluating the proposed quantitative measures of density change. The measures we developed are based on the Standard Mammogram Form (SMF) representation of interesting tissue and their performance (agreement with the expert’s description) is also compared to the “interactive thresholding” method that has been used in the past to characterise mammographic density. The results clearly indicate that present methods for measuring mammographic density fail to characterise temporal changes while the proposed measures have the potential to aid the radiologist in assessing temporal density changes both on a global and a local basis.

Mechanistic approaches for mixture risk assessments—present capabilities with simple mixtures and future directions

Mechanistic studies with simple mixtures have provided insights into the nature of interactions among chemicals that lead to non-additive effects and have elucidated the exposure conditions under which interactions are likely to occur. This paper discusses studies on four mixtures: (1) 1,1-dichloroethylene and trichloroethylene, (2) carbon tetrachloride and Kepone, (3) hexane and methyl- n-butylketone, and (4) coplanar and non-coplanar polychlorinated biphenyls. These mechanistic studies show that interactions should be described at the level of target tissue dose and are best categorized as either pharmacokinetic (PK) or pharmacodynamic (PD) interactions. In PK interactions the presence of a second chemical alters the kinetics such that a unit of administered dose no longer produces a unit of dose at the target tissue. In PD interactions, the presence of other compounds alters the PDs such that a unit tissue dose no longer produces a unit of response. Physiologically based pharmacokinetic (PBPK) models for mixtures have become important tools for predicting conditions under which interactions are likely to alter the assumption of additivity and have permitted calculation of interaction thresholds with more confidence. New cumulative risk assessment approaches have provided opportunities to classify compounds on the basis of similar chemistry-based modes of action (cholinesterase inhibitors) or similar physiological modes of action (diverse chemicals that alter a common biological outcome, such as defeminization of the developing nervous system). The latter examples present challenges for expanding our risk assessment paradigm to focus on the biology of responses more than on the kinetics of the xenobiotics. Some of the future advances in mixture research will depend on progress in systems biology, a discipline that integrates information across multiple level of biological organization producing PD models of normal function and assessing conditions under which exposures to chemicals lead to the perturbations sufficiently great to produce toxicity and disease. We describe briefly the elements of a systems biology approach for assessing the interactions between various PCB congeners.

Application of physiologically-based pharmacokinetic modeling to investigate the toxicological interaction between chlorpyrifos and parathion in the rat

Environmental exposure is usually due to the presence of multiple chemicals. In most cases, these chemicals interact with each other at both pharmacokinetic and pharmacodynamic toxicity mechanisms. In the absence of data, joint toxicity assessment of a mixture is based on default dose or response additivity. Although, the concept of additivity is mostly accepted at low dose levels, these levels need to be determined quantitatively to validate the use of additivity as an absence of any possible synergistic or antagonistic interactions at low environmental exposure levels. The doses at which interaction becomes significant define the interaction threshold. In most cases, estimation of these low-dose interaction thresholds experimentally is economically costly and challenging because of the need to use a large number of laboratory animals. Computational toxicology methods provide a feasible alternative to establish interaction thresholds. For example, a physiologically based pharmacokinetic (PBPK) model was developed to estimate an interaction threshold for the joint toxicity between chlorpyrifos and parathion in the rat. Initially, PBPK models were developed for each chemical to estimate the blood concentrations of their respective metabolite. The metabolite concentrations in blood out-put was then linked to acetylcholinesterase kinetics submodel. The resulting overall PBPK model described interactions between these pesticides at two levels in the organism: (a) the P450 enzymatic bioactivation site, and (b) acetylcholinesterase binding sites. Using the overall model, a response surface was constructed at various dose levels of each chemical to investigate the mechanism of interaction and to calculate interaction threshold doses. The overall model simulations indicated that additivity is obtained at oral dose levels below 0.08 mg/kg of each chemical. At higher doses, antagonism by enzymatic competitive inhibition is the mode of interaction.

PBPK modeling of the metabolic interactions of carbon tetrachloride and tetrachloroethylene in B6C3F1 mice

Potential exists for widespread human exposure to low levels of carbon tetrachloride (CT) and tetrachloroethylene (TET). These halocarbons are metabolized by the cytochrome P450 system. CT is known to inhibit its own metabolism (suicide inhibition) and to cause liver injury by generation of metabolically derived free radicals. The objective of this research was to use develop a physiologically based pharmacokinetic (PBPK) model to forcast the metabolic interactions between orally administered CT and TET in male B6C3F1 mice. Trichloroacetic acid (TCA), a stable metabolite of TET, was used as a biomarker to assess inhibition of the cytochrome P450 system by CT. Metabolic constants utilized for CT were 1.0 mg/kg/h for Vmaxc_CT and 0.3 for Km_CT (mg/l). Values for TET (based in TCA production), were 6.0 mg/kg/h for Vmaxc_TET was 3.0 mg/l for Km_TET. The rate of loss of metabolic capacity for CT (suicide inhibition) was describe as: Vmaxloss ( mg/ h)=− Kd ( RAM× RAM) , where Kd (h/kg) is a second-order rate constant, and RAM (mg/h) is the Michaelis–Menten description of the rate of metabolism of CT. For model simplicity, CT was assumed to damage the primary enzymes responsible for metabolism of CT (CYP2E1) and TET (CYP2B2) in an equal fashion. Thus, the calculated fractional loss of TET metabolic capacity was assumed to be equivalent to the calculated loss in metabolic capacity of CT. Use of a Kd value of 400 h/kg successfully described serum TCA levels in mice dosed orally with 5–100 mg/kg of CT. We report, for the first time, suicide inhibition at a very low dose of CT (1 mg/kg). The PBPK model under-predicted the degree of metabolic inhibition in mice administered 1 mg/kg of CT. This PBPK model is one of only a few physiological models available to predict the metabolic interactions of chemical mixtures involving suicide inhibition. The success of this PBPK model demonstrates that PBPK models are useful tools for examining the nature of metabolic interactions of chemical mixtures, including suicide inhibition. Further research is required to compare the inhibitory effects of inhaled CT vapors with CT administered by oral bolus dosing and determine the interaction threshold for CT-induced metabolic inhibition.

Patterns of visual field defects in chronic angle-closure glaucoma with different disease severity

Purpose To evaluate the patterns of visual field defects in patients with chronic angle-closure glaucoma (CACG) with varying extent of optic nerve damage. Design Prospective, consecutive, observational case series. Participants One hundred forty-six Asian patients with well-controlled CACG. Methods Visual field tests were performed using program 24-2 of the Humphrey Field Analyzer (Humphrey Instruments, San Leandro, CA) with the Swedish interactive thresholding algorithm standard. One hundred ten eligible visual fields were scored with the system adopted by the Advanced Glaucoma Intervention Study and were categorized into 4 groups accordingly: mild, moderate, severe, and end-stage. Each hemifield was divided into the nasal, paracentral, and arcuate areas, and field loss that involved respective areas was defined as nasal step, paracentral scotoma, and arcuate scotoma. Main outcome measures The distribution of field defect patterns in each group was evaluated. The mean deviation (MD) was compared among the 3 areas within one hemifield and between each pair of corresponding areas across the median raphe. Results The nasal area was the most commonly damaged area in the mild group, being noted in 52% of eyes in the superior hemifield and 58% of eyes in the inferior hemifield. In the moderate group, field loss involving both the nasal and arcuate areas dominated the superior hemifield, whereas field loss involving all three areas dominated the inferior hemifield. The MD of the nasal area was the worst among the three areas in each hemifield of the mild and moderate groups, as well as in the inferior hemifield of the severe group (all P < 0.001). There were no significant differences in the MD of each area between the superior hemifield and their inferior counterparts. However, the superior hemifield as a whole showed a better MD than the inferior hemifield ( P=0.034) in the mild group. Conclusions Visual field loss that involved the nasal area was the most common pattern in the early stage of CACG. The MD of the nasal area was worse than those of the arcuate and the paracentral areas within the same hemifield in the mild, moderate, and severe groups of CACG patients.

Sensitivity of Neurons to Weak Electric Fields

Weak electric fields modulate neuronal activity, and knowledge of the interaction threshold is important in the understanding of neuronal synchronization, in neural prosthetic design, and in the public health assessment of environmental extremely low frequency fields. Previous experimental measurements have placed the threshold between 1 and 5 mV/mm, although theory predicts that elongated neurons should have submillivolt per millimeter sensitivity near 100 microV/mm. We here provide the first experimental confirmation that neuronal networks are detectably sensitive to submillivolt per millimeter electrical fields [Gaussian pulses 26 msec full width at half-maximal, 140 microV/mm root mean square (rms), 295 microV/mm peak amplitude], an order of magnitude below previous findings, and further demonstrate that these networks are more sensitive than the average single neuron threshold (185 microV/mm rms, 394 microV/mm peak amplitude) to field modulation.

High‐Energy Neutrinos from Magnetars

Magnetars can accelerate cosmic rays to high energies through the unipolar effect and are also copious soft-photon emitters. We show that young, rapidly rotating magnetars whose spin and magnetic moment point in opposite directions emit high-energy neutrinos from their polar caps through photomeson interactions. We identify a neutrino cutoff band in the magnetar period-magnetic field strength phase diagram, corresponding to the photomeson interaction threshold. Within uncertainties, we point out four possible neutrino-emission candidates among the currently known magnetars, the brightest of which may be detectable for a chance on-beam alignment. Young magnetars in the universe would also contribute to a weak diffuse neutrino background, whose detectability is marginal, depending on the typical neutrino energy.

Can Swedish Interactive Thresholding Algorithm Fast Perimetry be used as an alternative to Goldmann perimetry in neuro-ophthalmic practice?

Can Swedish Interactive Thresholding Algorithm Fast Perimetry be used as an alternative to Goldmann perimetry in neuro-ophthalmic practice?

Full-threshold versus Swedish Interactive Threshold Algorithm (SITA) in normal individuals undergoing automated perimetry for the first time

Purpose To compare the full-threshold (FT) and Swedish Interactive Threshold Algorithm standard (SS) strategies in normal individuals undergoing automated perimetry for the first time. Design Randomized, comparative, observational case series. Participants Eighty perimetrically naive normal individuals. Methods All individuals underwent computerized visual field examinations (30-2, Humphrey 750, Humphrey-Zeiss, Dublin, CA) with both FT and SS strategies. One eye of each individual was tested. Test order between strategies was randomized. Main outcome measures The following variables were compared: test time, foveal threshold, fixation losses, false-positive and false-negative errors, mean deviation, pattern standard deviation, glaucoma hemifield test, and number of depressed points deviating at P < 5%, P < 2%, P < 1%, and P < 0.5% on the total and pattern deviation probability maps. Initially, we compared the results of all FT and SS tests, regardless of the order in which they were applied. Next, patients undergoing SS as the first examination were compared with those undergoing FT as the first test. Finally, the SS and FT results obtained in the second test were compared. Anderson’s criteria were applied to define abnormal examinations, allowing for the calculation of the specificity of the SS and FT strategies. Results When the results of all FT and SS tests were analyzed, the number of significantly depressed points deviating at P < 5%, P < 2%, and P < 1% on the pattern deviation probability maps was higher with the SS strategy ( P < 0.05); the specificity was 50% for SS and 72.5% for FT ( P < 0.01). When only first examinations were compared, the number of significantly depressed points deviating at P < 5%, P < 2%, and P < 1% on both the total and pattern deviation probability maps was higher with the SS strategy ( P < 0.05); the specificity was 38.1% and 63.2% for the SS and FT strategies, respectively ( P = 0.04). When only the second examinations were compared, there were no significant differences either between the number of depressed points or between the specificities of both strategies (73.7% for SS and 71.4% for FT) ( P = 0.98). Conclusions Normal individuals with no perimetric experience may present more significantly depressed points on the pattern deviation probability map when the SS strategy is used, reducing the test specificity in comparison with FT. These findings are probably due to a lower interindividual variability observed with SS. However, these differences disappeared in a second examination, suggesting that both strategies perform similarly in perimetrically experienced individuals.

Optical coherence tomography measurement of nerve fiber layer thickness and the likelihood of a visual field defect

PURPOSE: To determine if optical coherence tomography (OCT) measurements of nerve fiber layer (NFL) thickness can be used to predict the presence of visual field defects (VFD) associated with glaucoma. DESIGN: Quota-sampled, cross-sectional study. METHODS: Retrospective study of OCT NFL thickness measurements in 276 eyes of 276 subjects. All persons received OCT NFL thickness analysis; 136 eyes underwent frequency-doubling technology (FDT) perimetry; and 140 eyes underwent Swedish interactive threshold algorithm (SITA) perimetry. We defined a parameter called NFL 50, which is the NFL thickness value at which there was a 50% likelihood of a VFD with either SITA or FDT perimetry. We evaluated the use of NFL 50. RESULTS: The mean NFL thickness with (n = 68) and without (n = 68) a VFD in the FDT group was 93.2 μm ± 22.6 and 108.4 μm ± 14.1, respectively. The mean NFL thickness with (n = 70) and without (n = 70) a VFD in the SITA group was 78.9 μm ± 24.8 and 103.0 μm ± 18.0, respectively. The FDT mean NFL 50 value was 98.5 μm. The SITA mean NFL 50 value was 87.0 μm. The area under the receiver operator characteristic (AROC) curve for mean NFL was 0.73, and the positive predictive value (PPV) for FDT mean NFL 50 was 72.2%. For SITA mean NFL, the AROC was 0.79 and the PPV for NFL 50 was 77.2%. CONCLUSION: Nerve fiber layer thickness analysis using OCT may be clinically useful in identifying subjects who have visual field loss. However, the PPV suggests that OCT may need higher resolution and better reproducibility to enhance its sensitivity and specificity for population screening.