Performance of malaria RDTs on the sample with low parasite density
Among the 129 malaria RDT products analyzed in the summary of WHO product testing from round 5 to 8, there were 42 RDTs against Pf. The results showed that the PDS of nearly 12% products was less than 75%, while only 47.6% products displayed the PDS more than 90%. There were 48 RDTs against Pf and Plasmudium species (Pan), more than 20% ones showed the PDS < 75%, only 31.3% showed the PDS ≥ 90%. The results indicates that RDTs against Pf and other plasmodium species are less sensitive than those against Pf only. As for the RDT products to test Pv, 37.5% RDTs against Pf and Pan showed the PDS below 75%, only 31.3% products’ PDS ≥ 90%; 24.2% ones against Pf and Pv/Pvom showed the PDS < 75%, the PDS of 45.5% products was equal to and more than 90%. (Fig. 1)
Performance Of Malaria Rdts In Improper Rdt Storage
Thermal stability is a very important aspect of WHO's regular assessment for RDTs. Specifically, the detection rate of Pf was evaluated after malaria RDTs had been incubated at room temperature baseline (26℃) and room temperature, 35℃, and 45℃ for 60 days. [1, 3–9]. This paper summarizes the results of the 5-8th round of product assessment [12] as follows. (Fig. 2)
The results show that most of RDTs used to detect Pf can detect Pf 100% at both rather high temperatures of 35℃ and 45℃, the performance does not change with increase of temperature. As for RDT products detecting Pf and all plasmodium species and RDTs detecting Pf and Pv/P.ovale (Po) and malariae (Pm), the positivity rate decreases with increase of incubation temperature. In particular, a lot of RDT products that detect all plasmodium species did not achieve a 100% positivity rate at normal temperature and higher temperature. As a result, these products may generate FN results in testing blood sample with plasmodium.
Performance Of Malaria Rdts In Ease-of-use
For the aspect of ease of use, the blood safety and instruction quality were scored 0–3 and 0–2 respectively and put together to a combined score of 0–5 points. The higher the score, the more convenient the application. The results showed that the lowest score for all products is 2 and the highest is 5 (Fig. 3).
Table 1
Percentage distribution of test band intensity score against wild-type P.falciparum in 200 parasites/ul
Products Category
|
Percentage distribution of test band intensity score (100%)
|
0
|
1
|
2
|
3
|
4
|
Pf test band
|
|
|
|
|
|
Pf only (n = 44)
|
9.6 ± 12.0
|
19.4 ± 11.5
|
32.6 ± 9.0
|
23.8 ± 9.2
|
14.7 ± 11.2
|
Pf and pan(n = 48)
|
12.4 ± 10.9
|
22.6 ± 10.9
|
32.5 ± 10.1
|
22.7 ± 9.9
|
9.8 ± 9.0
|
Pf and Pv/Pvom (n = 33)
|
8.5 ± 4.8
|
19.6 ± 8.3
|
34.2 ± 6.0
|
26.7 ± 8.5
|
11.2 ± 7.7
|
Pf, Pf and Pv (n = 3)
|
16.8 ± 14.8
|
39.4 ± 33.4
|
22.3 ± 17.7
|
12.2 ± 17.2
|
9.5 ± 13.4
|
Pf, Pv and Pan (n = 1)
|
5.8
|
12
|
27.3
|
31.5
|
23.5
|
Pan test band
|
|
Pf and pan (n = 48)
|
52.9 ± 27.0
|
40.6 ± 21.7
|
6.5 ± 7.6
|
0.1 ± 0.2
|
0.0 ± 0.0
|
Pf, Pv and Pan (n = 1)
|
14
|
53.3
|
32
|
0.8
|
0
|
Pan only (n = 4)
|
6.0 ± 7.3
|
42.2 ± 24.1
|
33.6 ± 9.9
|
14.2 ± 16.2
|
4.1 ± 4.7
|
Pv test band
|
|
Pf and Pv/Pvom (n = 33)
|
95.6 ± 10.5
|
4.2 ± 10.0
|
0.3 ± 0.8
|
0.0 ± 0.0
|
0.0 ± 0.1
|
Pf, Pv and Pan (n = 1)
|
99.3
|
0.8
|
0
|
0
|
0
|
* P < 0.05 when compared among different category of malaria RDTs. |
Pf, Plasmudium falciparum; Pv, Plasmudium vivax; Pan, Plasmudium species; Pvom, Plasmudium vivax, ovale and malariae
Table 2
Percentage distribution of test band intensity score against wild-type P.vivax in 200 parasites/ul
Products Category
|
Percentage distribution of test band intensity score (100%)
|
0
|
1
|
2
|
3
|
4
|
Pf test band
|
|
|
|
|
|
Pf only (n = 39)
|
99.4 ± 1.3
|
0.6 ± 1.3
|
0.0 ± 0.0
|
0.0 ± 0.1
|
0.0 ± 0.0
|
Pf and pan (n = 32)
|
99.6 ± 0.9
|
0.3 ± 0.8
|
0.0 ± 0.2
|
0.1 ± 0.2
|
0.0 ± 0.0
|
Pf and Pv/Pvom (n = 29)
|
99.4 ± 1.4
|
0.6 ± 2.4
|
0.0 ± 0.1
|
0.0 ± 0.0
|
0.0 ± 0.0
|
Pf, Pf and Pv (n = 2)
|
100.0 ± 0.0
|
0.0 ± 0.0
|
0.0 ± 0.0
|
0.0 ± 0.0
|
0.0 ± 0.0
|
Pf, Pv and Pan (n = 1)
|
99.3
|
0.7
|
0
|
0
|
0
|
Pan test band
|
|
Pf and pan (n = 48)
|
13.1 ± 21.0
|
39.4 ± 24.0
|
42.3 ± 28.1
|
5.0 ± 7.5
|
0.3 ± 0.7
|
Pf, Pv and Pan (n = 1)
|
0
|
18.6
|
70.7
|
10
|
0.7
|
Pan only (n = 4)
|
2.7 ± 3.2
|
11.4 ± 7.2
|
65.9 ± 5.5
|
17.9 ± 5.9
|
2.1 ± 1.6
|
Pv test band
|
|
Pf and Pv/Pvom (n = 33)
|
6.9 ± 8.0
|
45.1 ± 22.5
|
42.6 ± 20.7
|
5.2 ± 10.6
|
0.2 ± 0.4
|
Pf, Pf and Pv (n = 1)
|
0.7
|
23.6
|
75.7
|
0
|
0
|
Pf, Pv and Pan (n = 1)
|
9.3
|
49.3
|
39.3
|
2.1
|
0
|
* P < 0.05 when compared among different category of malaria RDTs |
Pf, Plasmudium falciparum; Pv, Plasmudium vivax; Pan, Plasmudium species; Pvom, Plasmudium vivax, ovale and malariae
Merely 9.5% of Pf only products could obtain a score of 5, and 6.3% for Pf and pan, 6.1% for Pf and Pv/pvom achieved 5 point. On the contrast, a considerable proportion of products got the points less than or equal to 3, the percentage of ones for Pf was 26.2%, and for Pf and pan, Pf and Pv/pvom were 20.9%, 27.2% respectively.
Another aspect of ease of use is the intensity of the test band of the RDTs. 192 products and 133 test band results were included in this analysis. The band intensity graded as 0 (no visible band), 1,2,3 or 4 (1 being the weakest colour intensity and 4 the strongest) (Table 1, 2). When testing the Pf samples at low parasite density, 9.6 ± 12.0% of the Pf only products give one result with a bend intensity of 0, and 19.4 ± 11.5% give a band intensity of 1, then the average percentage of products with band intensity of 0 and 1 was about 30%. Meanwhile, Pf and Pan product has the highest proportion of band intensity of 0, approximately 35% of which obtain 0 and 1, and the different proportion of the 5 score ranges among different kinds of RDTs was insignificant statistically. When testing the Pf samples at low parasite density, intensity score of 0 and 1 account for about 90% in all pan bands of products for pf and pan. When testing Pv samples at the same parasite density, the proportion of RDTs for Pf and Pv/Pvom with intensity of 0 was 6.9 ± 8.0%, and that of 1 is 45.1 ± 22.5%. All the above data shows that there were considerable number of malaria RDTs give barely visible results, and having a higher risk of causing false negative results.
Performance Of Malaria Rdts Influenced By Lot-to-lot Variation
Consistency between test lots is calculated from the number of samples that return positive results on both RDTs tested in that lot against parasite-positive samples.
For low density Pf samples, The positive rate of two lots for Pf only products, Pf and Pan, pf and Pv/pvom were 88 ± 13% vs. 89 ± 12%, 84 ± 12% vs. 85 ± 14%, and 89 ± 6% vs. 89 ± 5%, respectively. For low density Pv samples, the positive rate of Pf and Pan, Pf and Pv/Pvom were 80 ± 27% vs. 80 ± 27% and 90 ± 12% vs. 89 ± 12% respectively. These comparisons suggest that a slight difference between two lots of all products in the assessment test, but there is no statistical significance. Nevertheless, it still indicated that different lots of one malaria RDTs product is likely to develop inconsistent results. (Fig. 4)
Performance of malaria RDTs against the Pf with deletion of the pfhrp2/3 gene
Round 8 of WHO product testing of malaria RDTs included the first comparative data on RDT performance for detection of P. falciparum with pfhrp2/3 gene deletions [10]. All RDTs were assessed against the panel that contained only low-parasite-density samples or samples with antigen concentrations comparable to 200 parasites/µL.
Figure 5 shows that the PDS of RDT products detecting HRP2-negative Pf varied considerably. Of the RDT products designed to detect Pf with Pf-LDH alone or in combination, the products of PDS < 50% accounted for 88.9%, ones of PDS ≥ 80% was nill, and the products with FNR ranged 20% − 50% accounted for 66.7% (Fig. 6). The two pan-LDH RDTs maintained high PDS of more than 80%, and the FNR both were < 20%. As for the 19 products designed to test for Pf with HRP2 only but that also had a non-Pf-LDH line, there were nearly 60% of RDTs with PDS < 20%, none with RDS ≥ 80%, and the FNR of all products were༞than 50%, 42.1% products were ≥ 80%. Therefore, most malaria RDT products had low sensitivity and high FNR against the HRP2-negative panel, even for products designed to detect Pf with pf-LDH and not HRP2.