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Abstract This prospective study investigated the magnitude and pattern of variation in choroidal optical coherence tomography angiography (OCT-A) indices every 4 h over 24 h in healthy young myopic (n = 24) and non-myopic (n = 20) adults. Choriocapillaris and deep choroid en-face images from macular OCT-A scans were analysed from each session to extract magnification-corrected vascular indices including choriocapillaris flow deficit number, size and density and deep choroid perfusion density in the sub-foveal, sub-parafoveal, and sub-perifoveal regions. Choroidal thickness was also obtained from structural OCT scans. Significant variations over 24 h (P

Purpose: To evaluate the repeatability of retinal and choroidal optical coherence tomography angiography (OCT‐A) indices among healthy children and compare it to healthy young adults. Methods: This prospective study captured 3 mm × 3 mm and 6 mm × 6 mm macular OCT‐A scans including superficial and deep retinal layers, choriocapillaris and deep choroid over two visits, 1 week apart at approximately the same time of day, for 22 healthy adults (18–30 years) and 21 children (6–15 years). Magnification and projection‐artefact corrected indices extracted using a custom image analysis program and individual biometry were compared between visits using Bland–Altman analysis and intraclass correlation (ICC). Retinal indices included foveal avascular zone metrics, perfusion and vessel density and choroidal indices included choriocapillaris flow deficit metrics and deep choroid perfusion density, in the foveal, parafoveal and perifoveal regions. Repeatability between adults and children was compared with F‐test. Results: Bland–Altman analysis showed that the mean differences between repeated OCT‐A indices were not significantly different from zero for either of the zones, layers and scan sizes in the two age groups (p > 0.05) except for foveal vessel density and foveal avascular zone perimeter (p = 0.04 for both) of 6‐mm‐deep retinal layer scans. The ICC ranged between 0.67 and 0.99. Significantly higher variability between visits (p < 0.05) in the indices was noted among adults than children, especially for choroidal indices of larger scan size. Conclusion: The retinal and choroidal OCT‐A indices in the foveal, parafoveal and perifoveal zones were repeatable in healthy children except for the foveal vessel density and foveal avascular zone perimeter of the 6‐mm‐deep retinal layer, which exhibited statistically borderline differences between visits. The adult group showed more variability between visits compared to children, especially in the larger scan size for choroidal OCT‐A indices. [ABSTRACT FROM AUTHOR]

Optical coherence tomography angiography (OCT-A) indices are likely to change across time and optometrists should be aware of the variability expected during childhood development and in healthy adults. Cross-sectional studies have shown that OCT-A indices are associated with age in adults and children. The aim of this study is to investigate longitudinal changes in retinal and choroidal OCT-A indices over 1 year among healthy children and young adults. This prospective longitudinal study captured macular OCT-A and OCT scans, and biometry measures at baseline and 1-year follow-up for 22 adults (18–30 years; −6.87 to +0.37 D) and 21 children (6–15 years; −5.75 to +0.25 D). Superficial and deep retinal layer, choriocapillaris and deep choroidal en face OCT-A images were analysed to extract magnification-corrected vascular indices in foveal, parafoveal and perifoveal regions. The retinal indices included foveal avascular zone metrics, perfusion, and vessel density. Flow deficit number, size, and density were extracted from choriocapillaris and perfusion density from deep choroid. Associations between annual changes in the OCT-A indices and axial length and baseline refraction were also studied. Among children, significant reductions were noted only in parafoveal superficial retinal and foveal and perifoveal deep retinal layer indices over 1 year (p < 0.05). Choroidal OCT-A indices in children and both retinal and choroidal OCT-A indices in adults did not show significant changes. Myopia was associated with a larger reduction in the perifoveal retinal OCT-A indices in children, and with increases in sub-foveal and sub-parafoveal choroidal indices in adults. There were associations between OCT-A indices and axial length changes but differently in adults and children. Significant changes were noted in retinal OCT-A indices over 1 year among children but not adults. In comparison, choroidal OCT-A indices in adults and children showed a stable morphology over this period of time. [ABSTRACT FROM AUTHOR]

Understanding devices used for vision screening, including their potential utilisation and validity, will facilitate proper utilisation of this technology. The Acuidrive is a self-illuminated, hand-held, visual acuity screening device intended for use in policing, with visual acuity assessed roadside to identify drivers who may not meet vision standards for driving. The target is presented binocularly at 24 cm, and +4.00 D lenses eliminate the accommodation requirement. This study investigates its validity and applicability relative to the Early Treatment of Diabetic Retinopathy Study (ETDRS) chart. There were 36 participants, half younger adults aged 18–30 years and half older adults aged 50–70 years. The subjects underwent binocular visual acuity testing using the Acuidrive device and an ETDRS chart displayed on a monitor. Eyes were corrected for distance using lenses in a trial frame, and additional trial lenses provided four blur levels: zero, +0.50 DS, +1.00 DS and +1.50 DS. Luminances for the devices were similar at approximately 100 cd/m2. ETDRS chart measurements were conducted both with room lighting on and off. The Acuidrive device underestimated the ETDRS visual acuity across all blur levels, with mean differences of 0.24 ± 0.07 logMAR and 0.18 ± 0.10 logMAR for room lights on and off, respectively. To predict ETDRS visual acuity of poorer than 0.34 logMAR (6/12=), a screening level of 0.50 logMAR (6/19) with the Acuidrive device provided high sensitivity and specificity (86% and 79% with lights-on, and 85% and 78% with lights-off). Visual acuity was better for the older group than the younger group by approximately 0.10 logMAR. There was an offset of 0.2 logMAR (two lines) between visual acuity measures for the Acuidrive device and an ETDRS chart. The Acuidrive device has the potential to be a viable screening tool with refinement to its construction. [ABSTRACT FROM AUTHOR]

Purpose: This study compared in vivo crystalline lens shape measurements using B‐scan images from the IOLMaster 700 with phakometry. Methods: Twenty‐four young adult participants underwent IOLMaster 700 and phakometry measurements under cycloplegia (1% cyclopentolate). The IOLMaster 700 generated B‐scan images along six meridians in 30° increments, which were analysed using custom MATLAB software to determine lens surface radii of curvature. Phakometry measurements were obtained using Purkinje images reflected from the lens surfaces. Results: The IOLMaster 700 image analysis method yielded a lower mean anterior lens surface spherical equivalent power (+6.20 D) than phakometry (+7.55 D); however, the two measurements were strongly correlated (R(21) = 0.97, p < 0.0001). The astigmatic power vectors (J0 and J45) for the anterior lens surface were significantly higher for the IOLMaster 700 measurements, with only J0 showing a significant moderate positive correlation (R(21) = 0.57, p = 0.005). For the posterior lens surface, the IOLMaster 700 measurements had a higher mean spherical power (+14.28 D) compared to phakometry (+13.70 D); however, a strong positive correlation (R(21) = 0.90, p < 0.0001) was observed. No significant correlations were noted for posterior lens surface astigmatic vectors (J0 and J45). The IOLMaster 700 estimates for the equivalent lens mean spherical power were slightly lower than those for phakometry, with a mean difference of −0.72 D, and both methods were positively correlated (R(21) = 0.94, p < 0.0001). Conclusions: The findings demonstrate that IOLMaster 700 B‐scan image analysis technique provides similar estimates of lens surface powers to phakometry. These results highlight the potential of the IOLMaster 700 to provide measurements of lens shape, informing future research and clinical use. [ABSTRACT FROM AUTHOR]

Binocular visual acuity is an important index of functional performance. Optometrists need to know how binocular visual acuity is affected by aniseikonia, and whether reduced binocular visual acuity is a marker for aniseikonia. Aniseikonia, the perception of unequal image sizes between the eyes, can occur spontaneously or can be induced after different types of eye surgery, or trauma. It is known to affect binocular vision, but there are no prior studies about how it affects visual acuity. Visual acuity was measured for 10 healthy well-corrected participants aged 18–21 years of age. Aniseikonia of up to 20% was induced in one of two ways: (1) size lenses, which provided minification of field of view in one eye of each participant and (2) polaroid filters, which allowed vectographic viewing of optotypes on a 3D computer monitor. The best corrected acuity was measured on conventional logarithmic progression format vision charts and isolated optotypes, under both induced aniseikonia conditions. Induced aniseikonia caused binocular visual acuity thresholds to increase by small but statistically significant amounts, with the largest deficit being 0.06 logMAR for 20% size differences between the eyes. Binocular visual acuity was worse than monocular visual acuity for aniseikonia of 9% and greater. Acuity measured with the vectographic presentation gave slightly higher thresholds (by 0.01 logMAR) than for those viewed with size lenses. Acuity measured with charts gave slightly higher thresholds (by 0.02 logMAR) than with isolated letters. An acuity change of 0.06 logMAR is small and may be missed in a clinical examination. Therefore, visual acuity cannot be used as a marker of aniseikonia in clinical settings. Even with very marked induced aniseikonia, binocular visual acuity remained well within standards for licen*c*sing of drivers. [ABSTRACT FROM AUTHOR]

Purpose: The Enright phenomenon describes the distortion in speed perception experienced by an observer looking sideways from a moving vehicle when viewing with interocular differences in retinal image brightness, usually induced by neutral density filters. We investigated whether the Enright phenomenon could be induced with monocular pupil dilation using tropicamide. Methods: We tested 17 visually normal young adults on a closed road driving circuit. Participants were asked to travel at Goal Speeds of 40 km/h and 60 km/h while looking sideways from the vehicle with: (i) both eyes with undilated pupils; (ii) both eyes with dilated pupils; (iii) with the leading eye only dilated; and (iv) the trailing eye only dilated. For each condition we recorded actual driving speed. Results: With the pupil of the leading eye dilated participants drove significantly faster (by an average of 3.8 km/h) than with both eyes dilated (p = 0.02); with the trailing eye dilated participants drove significantly slower (by an average of 3.2 km/h) than with both eyes dilated (p

The effect of vitamin A supplements on metabolic behavior of an oral tracer dose of [14C]β-carotene was investigated in a longitudinal test-retest design in two adults. For the test, each subject ingested 1 nmol of [14C]β-carotene (100 nCi) in an emulsified olive oil-banana drink. Total urine and stool were collected for up to 30 days; concentration-time patterns of [14C]β-carotene, [14C]retinyl esters, and [14C]retinol were determined for 46 days. On Day 53, the subjects were placed on a daily vitamin A supplement (10,000 IU/day), and a second dose of [14C]β-carotene (retest) was given on Day 74. All 14C determinations were made using accelerator mass spectrometry. In both subjects, the vitamin A supplementation was associated with three main effects: 1) increased apparent absorption: test versus retest values rose from 57% to 74% (Subject 1) and from 52% to 75% (Subject 2); 2) an ∼10-fold reduction in urinary excretion; and 3) a lower ratio of labeled retinyl ester/β-carotene concentrations in the absorptive phase. The molar vitamin A value of the dose for the test was 0.62 mol (Subject 1) and 0.54 mol (Subject 2) vitamin A to 1 mol β-carotene. Respective values for the retest were 0.85 and 0.74.These results show that while less cleavage of β-carotene occurred due to vitamin A supplementation, higher absorption resulted in larger molar vitamin A values.

A clinical study was conducted to investigate the dose response and metabolism of strawberry anthocyanins. In a crossover study design, 12 healthy adults consumed each of 3 strawberry treatments. The treatments were 100 g, 200 g, and 400 g of pureed strawberries, delivering 15[micro]mol, 30[micro]mol, and 60[micro]mol anthocyanin, respectively. Urine samples were collected for 24 h after each dose and samples were analyzed by HPLC with diode array detection and ion trap MS. Pelargonidin 3-glucoside was the major anthocyanin form in the treatments, and pelargonidin 3-glucoside and 3 metabolites of pelargonidin 3-glucoside (detected as monoglucuronides) were excreted in urine after ingestion. One predominant monoglucuronide form was detected in urine in masses 10-fold higher than the other 2 monoglucuronide forms. Increasing dose resulted in increasing appearance of anthocyanins in urine, and the mass of each pelargonidin monoglucuronide increased in urine with increasing dose. These results suggest that pelargonidin 3-glucoside absorption and metabolism are not saturated at masses [less than or equal to] 60[micro]mol, thus showing that more strawberry anthocyanin can be absorbed with increasing dose.

Corrinoid (vitamin [B.sub.12]-like) cofactors contain various [alpha]-axial ligands, including 5,6-dimethylbenzimidazole (DMB) or adenine. The bacterium Salmonella enterica produces the corrin ring only under anaerobic conditions, but it can form 'complete' corrinoids aerobically by importing an 'incomplete' corrinoid, such as cobinamide (Cbi), and adding appropriate [alpha]- and [beta]-axial ligands. Under aerobic conditions, S. enterica performs the corrinoid-dependent degradation of ethanolamine if given vitamin [B.sub.12], but it can make [B.sub.12] from exogenous Cbi only if DMB is also provided. Mutants isolated for their ability to degrade ethanolamine without added DMB converted Cbi to pseudo-[B.sub.12] cofactors (having adenine as an [alpha]-axial ligand). The mutations cause an increase in the level of free adenine and install adenine (instead of DMB) as an [alpha]-ligand. When DMB is provided to these mutants, synthesis of pseudo-[B.sub.12] cofactors ceases and [B.sub.12] cofactors are produced, suggesting that DMB regulates production or incorporation of free adenine as an [alpha]-ligand. Wild-type cells make pseudo-[B.sub.12] cofactors during aerobic growth on propanediol plus Cbi and can use pseudo-vitamin [B.sub.12] for all of their corrinoid-dependent enzymes. Synthesis of coenzyme pseudo-[B.sub.12] cofactors requires the same enzymes (CobT, CobU, CobS, and CobC) that install DMB in the formation of coenzyme [B.sub.12]. Models are described for the mechanism and control of [alpha]-axial ligand installation.

There is a need for an improved test of human ability to assimilate dietary vitamin [B.sub.12]. Assaying and understanding absorption and uptake of [B.sub.12] is important because defects can lead to hematological and neurological complications. Accelerator mass spectro metry is uniquely suited for assessing absorption and kinetics of carbon-14 ([sup.14]C)-Iabeled substances after oral ingestion because it is more sensitive than decay counting and can measure levels of [sup.14]C in microliter volumes of biological samples with negligible exposure of subjects to radioactivity. The test we describe employs amounts of [B.sub.12] in the range of normal dietary intake. The [B.sub.12] used was quantitatively labeled with [sup.14]C at one particular atom of the dimethylbenzimidazole (DMB) moiety by exploiting idiosyncrasies of Salmonella metabolism. To grow aerobically on ethanolamine,Salmonella enterica must be provided with either preformed [B.sub.12] or two of its precursors, cobinamide and DMB. When provided with [sup.14]C-DMB specifically labeled in the C2 position, cells produced [sup.14]C-[B.sub.12] of high specific activity (2.1 GBq/ mmol, 58 mCi/mmol) (1Ci = 37 GBq) and no detectable dilution of label from endogenous DMB synthesis. In a human kinetic study, a physiological dose (1.5[micro]g, 2.2 kBq/59 nCi) of purified [sup.14] C-B.sub.12] was administered and showed plasma appearance and clearance curves consistent with the predicted behavior of the pure vitamin. This method opens new avenues for study of [B.sub.12] assimilation. dimethylbenzimidazole | ethanolamine | metabolic engineering | Salmonella | Schilling test

Purpose: To investigate the effects of computer monitor pixel density on Landolt C visual acuity measurements and to provide appropriate pixel density recommendations for Landolt C vision chart design. Methods: Participants were 10 healthy observers aged 18–31 years (mean 21.7 ± 3.6). Logarithmic progression charts were used, consisting of eight rows of five Landolt C optotypes ranging from −0.40 logMAR to 0.30 logMAR. Monitor pixel angular subtense varied from 0.10 min arc to 1.97 min arc, achieved by changing the chart test distance. Testing took place with two filtering conditions: unfiltered optotypes, (pixels rendered either black or white) and anti‐aliased optotypes with pixel luminance averaged for a pixel square and rendered as grey levels. Results: Each participant's acuity versus pixel size data sets were fitted by a nonlinear relationship with acuity equal to an asymptotic threshold visual acuity (VAas) for small pixel sizes below a critical pixel size (Pcrit). For pixel sizes larger than Pcrit there was a linear relationship between acuity thresholds and pixel size. For anti‐aliased Landolt Cs, mean Pcrit was 1.23 min, and for unfiltered Landolt Cs average Pcrit was 0.65 min. For anti‐aliased LandoltCs, Pcrit was 2.01xVAas, and for unfiltered Landolt Cs Pcrit was 1.05xVAas. Conclusion: These results are quantitatively very similar to previous research on pixellation and Sloan optotype acuity. We have demonstrated that spatially filtering Landolt C optotypes acts as anti‐aliasing, to make them more robust to the degradation effects of pixellation. Previous recommendations for maximum pixel size on Sloan letter vision charts can be applied safely to Landolt C charts. [ABSTRACT FROM AUTHOR]

Purpose: This article aims to illustrate the principles of bearing fixing, a new computer‐controlled procedure for subjective determination of astigmatism, and to compare it with conventional clinician‐controlled Jackson crossed‐cylinder refraction. Methods: The principles and method for bearing fixing are described. Astigmatism was measured using bearing fixing and Jackson cross‐cylinder twice on 20 visually normal participants aged between 18 and 34 years. After final sphere adjustment, acuity measurements were made with each refraction estimate. Results: Bearing fixing results could be obtained for all participants. Mean bearing fixing cylinder magnitude was slightly higher than Jackson cross‐cylinder measures of cylinder magnitude, by 0.05D. Using vector analysis to take into account cylinder power and axis, mean bearing fixing astigmatism was not significantly different from cross‐cylinder astigmatism, but did have higher test‐retest variability (p < 0.05). Acuity with bearing fixing and cross‐cylinder corrections did not significantly differ in average value or repeatability. Conclusions: Bearing fixing is a feasible method for subjective determination of astigmatism, and there are a number of potential improvements to the method which could make it faster and more precise. [ABSTRACT FROM AUTHOR]

Stand magnifiers are still one of the most commonly prescribed classes of low vision devices. Their performance can be difficult to understand because stand magnifiers usually do not give an image at infinity. This review summarises the methods of describing image enlargement for stand magnifiers, emphasising their relationship to equivalent viewing distance (EVD). This is done in terms of the underlying optical equations, and measurement methods, and methods of prescribing. In the past, methods of determining EVD have been somewhat indirect, requiring accurate measurement of lens power, and image position. The use of digital photography provides an alternative, more direct, simpler method of determining EVD, which can be accomplished in-office. This method is described and it is demonstrated how it gives comparable results to older methods with small, clinically non-meaningful differences, that may be due to differences in image distance reference planes. Describing the performance of stand magnifiers in terms of their dioptric power, or in terms of 'nominal magnification' or 'trade magnification', is imprecise and misleading. It is better to use indices such as equivalent viewing power and EVD, which take into account the magnifier dioptric power, the image position of the magnifier and the distance a patient is from the magnifier. While EVD is a useful index for prescribing stand magnifiers, manufacturers do not always provide sufficient technical details to determine EVD for their stand magnifiers, and available tables of EVDs are more than a decade old and are likely to need updating. Photographic comparison provides a method for determining EVD, and this method can also be applied to other low vision devices. [ABSTRACT FROM AUTHOR]

The author, a college professor, cites specific instances in which his students misread examination questions. He considers his role in dealing with college students who have such poor reading ability. (SJL)

A valetudinarian is described to be third in rank after valedictorian and salutatorian. The complexities to understand the English language are explained.

Purpose: Ocular diurnal rhythms have been implicated in myopia, glaucoma, diabetes, and other ocular pathologies. Ocular rhythms have been well described in adults; however, they have not yet been fully examined in children. The goal of this study was to investigate ocular and systemic diurnal rhythms over 24 h in children. Methods: Subjects, ages 5 to 14 years (n = 18), wore a light, sleep, and activity monitor for one week to assess habitual sleep/wake patterns, then underwent diurnal measurements every 4 h for 24 h. Measurements included blood pressure, heart rate, body temperature, intraocular pressure (IOP), ocular biometry, and optical coherence tomography imaging. Saliva was collected for melatonin and cortisol analysis. Mean ocular perfusion pressure was calculated from IOP and blood pressure. Central corneal thickness, corneal power, anterior chamber depth, lens thickness, vitreous chamber depth, and axial length were determined from biometry. Total retinal thickness, retinal pigment epithelium (RPE) + photoreceptor outer segment thickness, photoreceptor inner segment thickness, and choroidal thickness were determined for a 1 mm diameter centred on the fovea. Subjects' amplitude and acrophase of diurnal variation for each parameter were determined using Fourier analysis, and mean acrophase was calculated using unit vector averaging. Results: Repeated measures analysis of variance (ANOVA) showed that all parameters except anterior chamber depth exhibited significant variations over 24 h (p ≤ 0.005 for all). Axial length underwent diurnal variation of 45.25 ± 6.30 μm with an acrophase at 12.92 h, and choroidal thickness underwent diurnal variation of 26.25 ± 2.67 μm with an acrophase at 1.90 h. IOP was approximately in phase with axial length, with a diurnal variation of 4.19 ± 0.50 mmHg and acrophase at 11.37 h. Total retinal thickness underwent a significant diurnal variation of 4.09 ± 0.39 μm with an acrophase at 15.04 h. The RPE + outer segment layer was thickest at 3.25 h, while the inner segment layer was thickest at 14.95 h. Melatonin peaked during the dark period at 2.36 h, and cortisol peaked after light onset at 9.22 h. Conclusions: Ocular and systemic diurnal rhythms were robust in children and similar to those previously reported in adult populations. Axial length and IOP were approximately in phase with each other, and in antiphase to choroidal thickness. These findings may have important implications in myopia development in children. [ABSTRACT FROM AUTHOR]

Aim: To evaluate feasibility, safety, and clinical efficiency of optometrists in conducting diabetic retinopathy screening and ocular health screening. Methods: This was a prospective observational trial study of newly developed optometric service pathways established at a community health clinic for diabetic retinopathy screening and a hospital ophthalmology clinic for ocular health screening. The study was carried out to assess the feasibility and safety of eye examinations conducted by optometrists. Patients were examined by optometrists using a standard eye examination at both clinics and re-examined by ophthalmologists as the reference standard. Optometrists recorded diagnoses of ocular conditions and classified referral urgency for each patient and these were compared with the diagnoses made by ophthalmologists, who were masked to the optometrists' findings. Results: There was a high concordance of 87.0% (95% CI 80.4%-93.6%) for the diagnoses between the optometrists and ophthalmologists. Of 26 patients considered by the optometrists to need ophthalmology referral, 23 were agreed as such by the ophthalmologists, giving good agreement, ê = 0.76 (95% CI 0.53 -0.94) between the optometrists and ophthalmologists on referral classification. Agreement by the ophthalmologists for referral urgency classifications (very urgent/urgent or non-urgent) was very good (K = 0.85, 95%CI 0.62-1.00). Conclusions: Ocular health examination by optometrists using optometric-eye care pathways is feasible and safe. Optometrists showed strong agreement with ophthalmologists when diagnosing patients who had a range of ocular conditions. Optometrists were also able to triage referrals and their urgency accurately, suggesting that they could play an extensive role as primary eye care providers, thus reducing unnecessary referrals to ophthalmology clinics. [ABSTRACT FROM AUTHOR]

Purpose: The effects of image doubling on logarithmic progression chart visual acuity were investigated by simulating diplopia on a computer monitor. Methods: Ten participants (6M, 4F) aged 21–28 years (mean 22.4 ± 2.3) were assessed viewing with their left eye while wearing their best correction. Stimuli were eight rows of five Sloan letters, from 0.4 to −0.3 logMAR (6/15 to 6/3) arranged in logarithmic progression format, generated on an HD monitor. Stimuli were generated with different magnitudes of doubling, different directions of doubling, and different intensity ratios between the ghost image and main images. Results: When the ghost image had the same intensity as the main image, there was a significant effect of doubling magnitude on visual acuity, with the mean acuity being −0.11 logMAR (6/4.7) for no doubling. Acuity thresholds (logMAR) increased when doubling exceeded 1 min of arc, reaching a level of 0.12 logMAR (6/7.9) for doubling of 16 min of arc. There were no significant effects of orientation on acuity, nor were there significant orientation‐doubling magnitude interaction effects of visual acuity. Image doubling magnitude level affected visual acuity differently for different ghost image intensities, with the highest acuity elevation occurring when ghost and main images were equal or nearly equal. For faint ghost image intensities (10% and 20%) image doubling did not significantly affect visual acuity. Conclusions: Image doubling will degrade visual acuity if doubling is greater than 1 min of arc, and ghost images are sufficiently intense. However, even with very obvious visually‐disturbing image doubling, visual acuity remains only slightly reduced. [ABSTRACT FROM AUTHOR]

Bland and Altman described approximate methods in 1986 and 1999 for calculating confidence limits for their 95% limits of agreement, approximations which assume large subject numbers. In this paper, these approximations are compared with exact confidence intervals calculated using two-sided tolerance intervals for a normal distribution. The approximations are compared in terms of the tolerance factors themselves but also in terms of the exact confidence limits and the exact limits of agreement coverage corresponding to the approximate confidence interval methods. Using similar methods the 50th percentile of the tolerance interval are compared with the k values of 1.96 and 2, which Bland and Altman used to define limits of agreements (i.e. [Formula: see text]+/- 1.96Sd and [Formula: see text]+/- 2Sd). For limits of agreement outer confidence intervals, Bland and Altman's approximations are too permissive for sample sizes <40 (1999 approximation) and <76 (1986 approximation). For inner confidence limits the approximations are poorer, being permissive for sample sizes of <490 (1986 approximation) and all practical sample sizes (1999 approximation). Exact confidence intervals for 95% limits of agreements, based on two-sided tolerance factors, can be calculated easily based on tables and should be used in preference to the approximate methods, especially for small sample sizes. [ABSTRACT FROM AUTHOR]