Assessing agreement between malaria slide density readings.
Assessing agreement between malaria slide density readings.
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DOI：
10.1186/1475-2875-9-4
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发表时间：
2010-01-04
期刊：
影响因子：
3
通讯作者：
Sutherland C
中科院分区：
文献类型：
--
作者：
Alexander N;Schellenberg D;Ngasala B;Petzold M;Drakeley C;Sutherland C
已经使用了几个标准来评估疟疾寄生虫密度的重复载玻片读数之间的一致性。这样的标准可以基于百分比差异、或绝对差异、或组合。在这两类标准之间作出选择的理由，以及选择界定可接受的一致性的差异幅度的理由，都不清楚。本文件寻求一种程序，避免目前这些选择的缺点，其参数值更明确的理由。预期载玻片内寄生虫密度的变化，即使是从均匀样品制备的。这就降低了灵敏度和观察者一致性的下限，由泊松分布量化。这意味着，如果使用固定百分比差异标准的标准来获得令人满意的一致性，则在低寄生虫密度下高估了差异读数的数量。采用固定绝对差的标准，在高寄生虫密度时也会发生同样的情况。对于理想载玻片，遵循泊松分布，基于平方根计数的恒定差异的标准适用于所有密度。这可以反向转换为绝对计数的差异，正如预期的那样，在较高的平均密度下，这给出了更宽范围的可接受一致性。在坦桑尼亚的一个示例数据集中，观察到的平方根计数差异对应于平均密度为2，000个寄生虫/μl时-2，800和+2，500个寄生虫/μ l以及10，000个寄生虫/μ l时-6，200和+5，700个寄生虫/μ l的95%一致性限度。然而，在更高的密度下，有更多的离群值超出这些范围，这意味着这些范围的实际覆盖率不是恒定的95%，而是随着密度而下降。在第二项研究中，一项显微镜培训试验，相应的一致性范围更宽且不对称：分别为-8，600至+5，200/μl和-19，200至+11，700/μl。相比之下，对应于泊松变异的最佳一致性限度分别为± 780和± 1，800个寄生虫/μl。这种方法对血液读取量的关注导致了其他结论。例如，无论读取多大体积的血液，某些密度都太低而不能可靠地检测，这又意味着载玻片阳性的不一致可能仅仅是由于载玻片内变化而不是阅读错误。所提出的方法定义了可接受的协议的方式，允许自然增加的变化与寄生虫密度的限制。这包括定义与随机变化一致的读者之间的差异性水平：在这些限制内的差异不应触发额外的读数。这种方法值得在其他情况下进行调查，以确定其适用范围和协议限度的适当数值。
Several criteria have been used to assess agreement between replicate slide readings of malaria parasite density. Such criteria may be based on percent difference, or absolute difference, or a combination. Neither the rationale for choosing between these types of criteria, nor that for choosing the magnitude of difference which defines acceptable agreement, are clear. The current paper seeks a procedure which avoids the disadvantages of these current options and whose parameter values are more clearly justified. Variation of parasite density within a slide is expected, even when it has been prepared from a homogeneous sample. This places lower limits on sensitivity and observer agreement, quantified by the Poisson distribution. This means that, if a criterion of fixed percent difference criterion is used for satisfactory agreement, the number of discrepant readings is over-estimated at low parasite densities. With a criterion of fixed absolute difference, the same happens at high parasite densities. For an ideal slide, following the Poisson distribution, a criterion based on a constant difference in square root counts would apply for all densities. This can be back-transformed to a difference in absolute counts, which, as expected, gives a wider range of acceptable agreement at higher average densities. In an example dataset from Tanzania, observed differences in square root counts correspond to a 95% limits of agreement of -2,800 and +2,500 parasites/μl at average density of 2,000 parasites/μl, and -6,200 and +5,700 parasites/μl at 10,000 parasites/μl. However, there were more outliers beyond those ranges at higher densities, meaning that actual coverage of these ranges was not a constant 95%, but decreased with density. In a second study, a trial of microscopist training, the corresponding ranges of agreement are wider and asymmetrical: -8,600 to +5,200/μl, and -19,200 to +11,700/μl, respectively. By comparison, the optimal limits of agreement, corresponding to Poisson variation, are ± 780 and ± 1,800 parasites/μl, respectively. The focus of this approach on the volume of blood read leads to other conclusions. For example, no matter how large a volume of blood is read, some densities are too low to be reliably detected, which in turn means that disagreements on slide positivity may simply result from within-slide variation, rather than reading errors. The proposed method defines limits of acceptable agreement in a way which allows for the natural increase in variability with parasite density. This includes defining the levels of between-reader variability, which are consistent with random variation: disagreements within these limits should not trigger additional readings. This approach merits investigation in other settings, in order to determine both the extent of its applicability, and appropriate numerical values for limits of agreement.
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