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Variability in measuring pH using litmus paper and the relevance in ocular chemical injury
Eye ( IF 3.9 ) Pub Date : 2019-12-17 , DOI: 10.1038/s41433-019-0737-z
Mary-Therese Monaghan 1 , Kerr Brogan 1 , David Lockington 1 , Alan Paul Rotchford 1 , Kanna Ramaesh 1
Affiliation  

Emergency management of ocular chemical injuries require copious irrigation to dilute and remove the damaging agent. The quantity of irrigation is influenced by the initial ocular pH. Inaccurate pH measurements derived from litmus paper have been reported, and it has been our observation that litmus paper available is often aged and light exposed [1]. We wished to investigate inter-user variability in pH measurements using litmus paper. A total of 22 participants (five ophthalmology trainees, 11 ophthalmic nurses, and six junior doctors) were recruited and asked to assess the pH of five different solutions. The tip of new litmus paper was submerged for 2 s, then correlated with the reference colour chart after 30 s. Intra-class correlation coefficient was 0.753 (ANOVA), suggesting statistically similar values were obtained. However, pH readings differed by 1.69 pH units 5% of the time (95% CI 1.50–1.89) and measurements by two different individuals disagreed by more than 2.39 pH units 5% of the time (95% CI 2.11–2.67). Tear film pH has diurnal variation, with more acidic measurements following prolonged eye closure. Tear pH can be alkaline in keratitis (7.14 ± 0.28), keratoconjunctivitissicca, and corneal transplantation, and acidic in contact lens wearers (6.66 ± 0.28) [2]. Litmus paper is cheap and readily available, but there are occasions when it’s accuracy is questionable. Light exposure can bleach the reference colour scale (Fig. 1a, b), and damp environments can influence the quality of the paper (Fig. 1c), while colour deficiency traits can influence an individual’s ability to differentiate the scale [1, 3]. In 471 gastric fluid samples analysis found correlation between litmus paper and a pH meter was moderate (0.59; P < 0.001) but the concordance was low (0.54) [4]. Studies of pH in 42 pleural fluid samples found significant variability with litmus paper compared with blood gas analyser (coefficient of variation 7.5%) [5]. Although there was a strong correlation of pH measurements in our study, 5% of the results were unreliable and could adversely influence clinical management. As blood gas analysers are available in most major casualty departments, we recommend their input to confirm abnormal pH measurements derived from litmus paper in ocular chemical injuries not responding to the initial management of irrigation.

中文翻译:

使用石蕊试纸测量 pH 值的变异性与眼部化学损伤的相关性

眼部化学损伤的紧急处理需要大量冲洗以稀释和去除有害物质。冲洗量受初始眼部 pH 值的影响。据报道,从石蕊试纸获得的 pH 值测量不准确,我们观察到可用的石蕊试纸经常老化并暴露在光线下 [1]。我们希望使用石蕊试纸研究用户间 pH 值测量的变异性。总共招募了 22 名参与者(5 名眼科实习生、11 名眼科护士和 6 名初级医生)并要求评估五种不同溶液的 pH 值。将新石蕊试纸的尖端浸入水中 2 秒,然后在 30 秒后与参考比色图相关联。类内相关系数为 0.753 (ANOVA),表明获得了统计上相似的值。然而,pH 读数在 5% 的情况下相差 1.69 个 pH 单位(95% CI 1.50–1.89),并且两个不同个体的测量值在 5% 的情况下相差超过 2.39 个 pH 单位(95% CI 2.11–2.67)。泪膜 pH 值具有昼夜变化,长时间闭眼后酸性测量值更高。角膜炎 (7.14 ± 0.28)、角结膜炎和角膜移植的泪液 pH 值可能呈碱性,而隐形眼镜佩戴者的泪液 pH 值可能呈酸性 (6.66 ± 0.28) [2]。石蕊试纸便宜且容易获得,但有时其准确性值得怀疑。光照会漂白参考色标(图 1a、b),潮湿环境会影响纸张质量(图 1c),而色差特征会影响个人区分色标的能力 [1, 3] . 在 471 份胃液样本分析中发现,石蕊试纸与 pH​​ 计之间的相关性为中等 (0.59;P < 0.001),但相关性较低 (0.54) [4]。对 42 份胸水样本的 pH 值研究发现,与血气分析仪相比,石蕊试纸存在显着差异(变异系数为 7.5%)[5]。尽管在我们的研究中 pH 测量值有很强的相关性,但 5% 的结果不可靠并且可能对临床管理产生不利影响。由于大多数主要伤员部门都有血气分析仪,我们建议他们输入以确认来自眼化学损伤中石蕊试纸的异常 pH 测量值,对初始冲洗管理没有反应。对 42 份胸水样本的 pH 值研究发现,与血气分析仪相比,石蕊试纸存在显着差异(变异系数为 7.5%)[5]。尽管在我们的研究中 pH 测量值有很强的相关性,但 5% 的结果不可靠并且可能对临床管理产生不利影响。由于大多数主要伤员部门都有血气分析仪,我们建议他们输入以确认来自眼化学损伤中石蕊试纸的异常 pH 测量值,对初始冲洗管理没有反应。对 42 份胸水样本的 pH 值研究发现,与血气分析仪相比,石蕊试纸存在显着差异(变异系数为 7.5%)[5]。尽管在我们的研究中 pH 测量值有很强的相关性,但 5% 的结果不可靠并且可能对临床管理产生不利影响。由于大多数主要伤员部门都有血气分析仪,我们建议他们输入以确认来自眼化学损伤中石蕊试纸的异常 pH 测量值,对初始冲洗管理没有反应。
更新日期:2019-12-17
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