Assessment of patient complexity using routinely collected data: The UK CHIC study
Authors: Sabin C , Delpech V ,   UK Collaborative HIV Cohort (CHIC) Study
Published: November 2012

We identified predictors of clinical complexity based on data collected in the UK CHIC Study. All subjects under established care (>1 year) from 2000–2010 were included. Each subject's follow‐up (1 year after study entry to last clinic visit, death or 31/12/2010) was stratified into a series of 6–monthly periods and his/her status was assessed at the start of each. Using Poisson regression (with generalised estimating equations to allow for multiple entries per subject), we studied associations between demographic/clinical factors, CD4 count/percent, viral load (VL), calendar year and measures of prior/current antiretroviral (ART) use, and the development of a new AIDS event or death during each period. A complexity score was derived from the coefficients of the final model; subjects were categorised into ten equally sized groups based on the score, and event rates were calculated for each group. The 31,338 eligible subjects had a median (interquartile range) age of 36 (10, 42) years at baseline. Ethnicity was white (55%), black African (27%), black other (5%), other (9%) and unknown (4%). Mode of acquisition was sex between men (52%), heterosexual sex (37%), other (5%) and unknown (5%). Subjects contributed a total of 377,284 periods of follow‐up (181,170 person‐years [PY]) of which 5796 included a clinical event (rate/1000 PY: 3.20 [95% confidence interval 3.12, 3.28], 4322 AIDS events, 1534 deaths). As an active AIDS‐defining event in the past 6 months was the dominant predictor of a new clinical event (relative rate 41.55), subjects with an active event were excluded from further analysis. Risk factors for a clinical event in patients without an active AIDS event (Table 1) were earlier calendar year, non‐white ethnicity, older age, lower CD4 count,>80 CD4 cell drop from previous visit, being off ART or on ART with a VL>10,000 copies/ml.

Estimates from multivariable Poisson regression model of factors associated with a new clinical event over subsequent 6‐month period (excluding patients with an active AIDS event)

RR*
(95% CI)
P‐value

Calendar year / later year
0.97
(0.96, 0.98)
0.0001

Ethnicity

–

White
1
–
–

Black African
1.23
(1.13, 1.34)
0.0001

Black other
2.34
(2.06, 2.65)
0.0001

Other
1.64
(1.24, 2.17)
0.0001

Unknown
1.64
(1.38, 1.94)
0.0006

Age (years)

<30
0.59
(0.51, 0.69)
0.0001

≥30,<35
0.72
(0.64, 0.80)
0.0001

≥35,<40
0.75
(0.69, 0.83)
0.0001

≥40,<45
0.84
(0.76, 0.92)
0.0001

≥45
1
–
–

CD4 count (cells/mm3
)

<200
2.49
(2.25, 2.75)
0.0001

200–349
1
–
–

350–499
0.72
(0.65, 0.80)
0.0001

≥500
0.51
(0.46, 0.57)
0.0001

Unknown
0.71
(0.54, 0.93)
0.01

>6 Months of immune suppression
0.84
(0.76, 0.93)
0.001

>80 Cell drop in CD4 count from previous visit
1.15
(1.05, 1.27)
0.004

Previous (non‐active) AIDS event
2.51
(2.31, 2.74)
0.0001

Viral load/HAART status

On HAART VL<1000 c/ml
1
–
–

On HAART VL≥1000 c/ml
1.81
(1.62, 2.02)
0.0001

On HAART VL≤10000 c/ml
1.23
(1.10, 1.38)
0.0002

On HAART VL>10000 c/ml
2.61
(2.38, 2.86)
0.0001

Missing viral load
2.00
(1.55, 2.56)
0.0001

Hepatitis B/C co‐infection
0.81
(0.73, 0.89)
0.0001

Hepatitis co‐infection and previous experience of immune suppression were associated with lower clinical risk. A score based on this model discriminated reasonably well between subjects who did/did not develop an endpoint over the next 6 months (approximate C‐statistic: 0.72), with event rates increasing from 0.49/100 PY in the lowest score group to 7.16/100 PY in the highest. A score based on clinical markers may provide a means to identify those who will experience clinical progression over the next 6 months, allowing this group to be targeted for closer monitoring and funds for HIV care to be distributed appropriately.