TABLE 2.

Effects of patient age and viral load on DFA sensitivity for S-OIV^a

Range of swine A influenza virus PCR CT valuesb	No. of samples in range (n = 272)	No. of cytospin-DFA-positive samples/no. of samples tested (%) by patient age and CT value
		<5 yrs	5-17 yrs	≥18 yrs
12-19	113	13/13 (100)	58/58 (100)	42/42 (100)
20-25	102	10/10 (100)	56/61 (91.8)	29/31 (93.6)
26-30	43	2/3 (66.7)	11/25 (44.0)	6/15 (40.0)
31-35	9	0/1 (0)	2/6 (33.3)	1/2 (50.0)
36-40	5	0	0/2	0/3 (0)
Total no. of positive samples/total no. of samples tested (%)c	230/272 (84.6)	25/27 (92.6)	127/152 (83.5)	78/93 (83.8)

^aAbbreviations: S-OIV, swine origin influenza virus; C_T, cycle threshold, the amplification cycle at which the real-time PCR result crosses the threshold to positive (the lower the C_T, the higher the viral load present in the sample).

^bThe PCR C_T value is a reflection of viral load. Seven samples positive by pan A PCR only were excluded; all were DFA negative. Eight samples positive by pan A and swine A PCR but negative by swine H1 PCR were included; 7 were DFA negative and one was DFA indeterminate.

^cOf samples with swine A PCR C_T of <26, 208/215 (96.7%) were DFA positive. Of samples with a C_T of ≥26, 22/57 (38.6%) were DFA positive.