Background: A key element to consider when predicting future spread of resistant HIV is the proportion of treated people with moderate to high levels of virus which carries resistance mutations. We sought to estimate this for our large, free access clinic in London.

Methods: We have previously reported trends in the number and proportion of treated people with viral load > 400 c/mL. We used data on resistance testing to estimate the proportion of these with various levels of drug resistance. Genotypic resistance was determined using TRUGENE HIV-1 test (Bayer Diagnostics).

Results: Approximately 1,100–1,200 patients (pts) have been under regular follow-up at any one time, with 70% (713/1,021) on ART by 2001 (68% 694/1,021 on HAART and 59% 604/1,021 on HAART ³ 30 weeks [wks]). We have previously reported that the proportion of ART-treated people (on HAART for ≥ 30 wks) with viral load > 400 copies/mL has decreased from 21% (n = 88/417) in 1999 to 12% (n = 72/604) in 2001. These people may not all harbour resistant virus, due to poor adherence or other factors. The table below shows data on results from resistance tests performed on people on ART since Jan 1998.

 

Year(s)	N	# drug classes with mutations (> 1 IAS-USA mutation—excl. minor protease)				Med. viral load
		0	1	2	3
1998	106	14%	35%	44%	7%	4.8 log
1999	65	12%	32%	45%	11%	4.4 log
2000	75	12%	31%	45%	12%	4.2 log
2001+ (+ to Nov)	47	15%	36%	36%	13%	3.8 log

 

Conclusions: There has been no increase in use of resistance testing in treated people in our clinic since 1998, consistent with the low and decreasing proportion of ART-treated people with viral load > 400 c/mL. The proportion of those tested showing resistance to 1/2/3 classes has remained relatively stable over time. Only around 12%–15% of those on ART in whom resistance tests were performed showed no resistance mutations, suggesting (assuming the same proportion with resistance in those not tested) that around 88% of the 72 pts with viral load > 400 c/mL in 2001 carried some resistance. This suggests around 10% (63/604) of treated people in our clinic have resistant virus and viral load > 400 c/mL.