Inter-observer agreement according to malaria parasite density

Abstract

BackgroundRecent developments in diagnostic techniques for malaria, particularly DNA probes and sero-immunology, have raised questions as to how these techniques might be used to facilitate malaria diagnosis at the most peripheral levels of the primary health care system. At present, malaria diagnosis is based on the standard microscopic examination of blood films in most field epidemiologic studies and is likely to remain so in the immediate future in Africa. The objective of this study was to assess inter-observer agreement for the examination of Giemsa-stained slides for Plasmodium falciparum parasites.MethodsChildren aged 0 to 10 years were enrolled yearly in Bancoumana village (West Africa), mainly during the transmission season (June to October). The blood smears obtained from the persistently negative children in June 1996, August 1996, October 1996 and March 1997 were systematically re-examined. A stratified random sample (10%) proportional to the following parasite density classes 1–100, 101–5000, and 5001 and over was taken from the slides collected. The kappa statistics and the intra-class correlation were used as measures of agreement the first and the second slide examinations.ResultsThe weighted kappa statistic, widely used as a chance-corrected measure for nominal agreement, showed excellent inter-observer agreement (κw=0.7926; 95% CI [0.7588, 0.8263]; p=0.01). The intra-class correlation co-efficient had the same value of 0.7926 confirming the appropriateness of the weighted kappa statistic. Inter-observer agreement for slides read as negative by one observer, or as containing more than 100 parasites per μl, was excellent: 97% (493/506) and 92% (145/158), respectively. In contrast, the inter-observer agreement for slides read by one observer as containing 1–100 parasites/μl was poor, 36% (96/268).ConclusionsIn field conditions in Mali, there was a high reproducibility for slides reported as negative or as having more than 100 parasites per μl. However, smears with readings of 1–100 parasites per μl were less reproducible and should be re-examined carefully.

Different patterns ofpfcrtandpfmdr1polymorphism inPlasmodium falciparumisolates from Tehama region, Yemen

“…Parasite species and stages were recorded and parasitaemia (parasite density) was determined by counting only the asexual stages against 300 white blood cells (WBCs) and then multiplying by 25, assuming the average of the total WBC count of the individuals was equal to 7,500 cells per µl of blood ( Billo et al , 2013 ). The level of parasitaemia was graded as low (<1,000 parasites/µl of blood), moderate (1,000–9,999 parasites/µl of blood), or severe (≥10,000 parasites/µl of blood).…”

Diagnostic accuracy of loop-mediated isothermal amplification (LAMP) for screening patients with imported malaria in a non-endemic setting

“…But microscopy is recommended for malaria when its quality can be maintained and strong expertise is available. Unfortunately, in endemic and non-endemic countries, expert malaria microscopists are uncommon in non-specialized clinical laboratories and impaired microscopy-based diagnosis, especially for non- falciparum species, can be suspected [ 1 ]. The limit of detection (LOD) may vary substantially depending on the experience and training of microscopists, with a range of 5 to 100 parasites/μL [ 31 ].…”

Assessment of the inter-rater reliability of the microscopic diagnosis of malaria in three Health Centres of Kayonza District, Eastern Province, Rwanda

“…A few studies have shown that the malaria prevalence, estimated as the proportion of stained blood slides found to have malaria parasites, tend to vary considerably at low parasite densities ranging between 4 -100/μl of blood, but much less so above that density (Billo et al, 2013;Harris et al, 2010;McKenzie, Sirichaisinthop, Miller, Gasser, & Wongsrichanalai, 2003). There are some known factors that adversely affect the slide reading results at low parasite density levels.…”

“…There are some known factors that adversely affect the slide reading results at low parasite density levels. These include quality of slide preparation, quality of reagents and microscopes, random selection of microscope fields, level of training, and motivation and concentration including workload of the microscopists as well as to whether context is clinical or research (Billo et al, 2013;Harris et al, 2010;McKenzie et al, 2003). For example, the malaria microscopist technologists located in the malaria endemic areas are faced with large numbers of blood slides to read daily (Tangpukdee, et al, 2009), which tend to decrease their ability to perform accurately and with precision (World Health Organzation, 2009).…”