Description and Prediction of Physical FunctionalDisability in Psoriatic Arthritis: A LongitudinalAnalysis Using a Markov Model ApproachJANICE A. HUSTED,1 BRIAN D. TOM,2 VERNON T. FAREWELL,2 CATHERINE T. SCHENTAG,3

AND DAFNA D. GLADMAN3

Objective. To describe the longitudinal course of physical functioning in patients with psoriatic arthritis.Methods. Between June 1993 and June 2003, 341 patients attending the University of Toronto Psoriatic Arthritis Cliniccompleted 2 or more Health Assessment Questionnaires (HAQs). At the time of administration of each HAQ, patients wereassigned to 1 of 3 physical functional disability states, based on their HAQ score. A Markov model that allowed fortransitions to and from these 3 disability states was used to characterize the longitudinal course of physical functioning,as well as to identify factors for both progression and regression of disability.Results. Despite patient variability in the course of physical functioning, the following 3 longitudinal patterns wereobserved: 1) a stable state of disability throughout the entire study period, with 28%, 12%, and 6% of patients experiencingno, moderate, or severe disability, respectively; 2) a steady improvement or deterioration in disability over time (thispattern was observed in 27% of patients); and 3) a fluctuating state of disability, occurring in 27% of the patients. Sex, age,disease duration, number of actively inflamed joints, and number of deformed joints predicted transitions betweendisability states.Conclusion. Although 28% of patients appeared resistant to becoming disabled over the duration of this study, theremaining patients were observed either to experience enduring disability or to move between disability states.

KEY WORDS. Psoriatic arthritis; Physical functional disability; longitudinal analysis.

INTRODUCTION

Psoriatic arthritis (PsA) is an inflammatory arthritis that isassociated with psoriasis; patients are usually seronegativefor rheumatoid factor. Prior to the mid 1980s, PsA wasconsidered a benign disease, with short-lived synovitisthat did not lead to residual damage in most patients.Since 1987, however, we and other investigators haveshown that in many patients with PsA, deformities, dam-age, and disease progression develop over time (1–8). Fur-

thermore, cross-sectional studies have shown that physi-cal functioning among patients with PsA is significantlylower than that among healthy controls and comparablewith that of patients with rheumatoid arthritis (RA) (9–11). However, it is unknown whether the physical limita-tions observed cross-sectionally reflect short-lived, epi-sodic, or chronic disability. From a clinical perspective, itwould be useful to understand the pattern of physicaldisability over time, with particular attention to the factorsassociated with persistent or chronic disability.

The aim of this study was to describe the course ofphysical functional disability in PsA. Although it is ex-pected that some patients will experience a steady declinein function over time, it is also expected that others willexperience either steady improvement or a highly variablecourse. To capture all of these possible changes over time,we adopted a reversible multistate Markov model thatallows for transitions to and from physical functional dis-ability states (12). Although Markovian models have beenused to study transitions between functional states amongelderly residing in the community (13), thus far they havenot been used to study similar changes in arthritis popu-lations.

Supported by the Canadian Institute of Health Researchand the Krembil Foundation.

1Janice A. Husted, PhD: University of Waterloo, Waterloo,Ontario, Canada; 2Brian D. Tom, PhD, MRC, Vernon T.Farewell, PhD: Institute of Public Health, Cambridge, UK;3Catherine T. Schentag, MSc, Dafna D. Gladman, MD,FRCPC: University of Toronto, University Health Network,Toronto Western Hospital, Toronto, Ontario, Canada.

Address correspondence to Dafna D. Gladman, MD,FRCPC, Centre for Prognosis Studies in Rheumatic Disease,Toronto Western Hospital, 399 Bathurst, ECW 5-034B, To-ronto, Ontario, M5T 2S8, Canada. E-mail: dafna.gladman@utoronto.ca.

Submitted for publication May 26, 2004; accepted in re-vised form December 17, 2004.

Arthritis & Rheumatism (Arthritis Care & Research)Vol. 53, No. 3, June 15, 2005, pp 404–409DOI 10.1002/art.21177© 2005, American College of Rheumatology

ORIGINAL ARTICLE

404

PATIENTS AND METHODS

Patient population. Established in 1978, the Universityof Toronto PsA clinic at the Toronto Western Hospitalrepresents the largest population of patients with PsA evercollected and followed prospectively. It is both a primary,secondary, and tertiary referral center, including patientswith mild to severe disease as well as patients with newlydiagnosed disease and those with existing disease.

Patient assessment at first clinic visit and followup. Ateach visit, a complete history was obtained and an exam-ination was performed according to a standard protocol,and data were recorded according to data retrieval proto-col. Briefly, inquiry was made into demographic features(e.g., age, marital status, and education), the presence ofjoint inflammation, medications, the American College ofRheumatology (formerly the American Rheumatism Asso-ciation) functional level (14), and general medical history.The examination consisted of a general medical examina-tion with particular attention to the skin, nails, eye, andheart, as well as both peripheral and axial joints. Psoriasisseverity was rated according to the Psoriasis Area andSeverity Index (PASI) (15). The number of actively in-flamed joints (stress pain, joint line tenderness, and effu-sion) and deformed joints (ankylosis, subluxation, or de-creased range of motion �20%, attributable to jointdamage rather than inflammation), were recorded (16).Patients were followed at regular intervals (6–12 months).

Since June 1993, the Health Assessment Questionnaire(HAQ) (17) has been administered annually to clinic pa-tients. The HAQ assesses physical functional status overthe past week and includes questions related to fine move-ments of the upper extremity, locomotor activities of thelower extremity, and activities that involve both upper andlower extremities. The HAQ consists of 20 questions thatcover 8 categories of daily living (i.e., dressing and groom-ing, arising, eating, walking, hygiene, reach, grip, and ac-tivities, including errands and chores). Patients rate theirability to perform a particular task within a category on ascale from 0 (no difficulty) to 3 (unable to do). The highestscore for any task within a category determines the scorefor that category, with an adjustment for the use of devicesor the need for assistance. The scores for all 8 categoriesare then averaged to obtain an overall score on a scale from0 (no disability) to 3 (severe disability). The HAQ has beenshown to be reliable in our clinic population (18–20).

Statistical methods. Overall HAQ scores were dividedinto 3 categories: 0–0.49, 0.5–1.5, and 1.51–3, represent-ing physical functional disability states 1, 2, and 3, respec-tively. State 1 represented the absence of disability, whilestates 2 and 3 represented “moderate” and “severe” dis-ability, respectively. These cut-offs were consistent withpast studies, where HAQ scores less than 0.50 representedno disability (i.e., few difficulties in performing daily ac-tivities), and scores above 1.50 reflected a higher or moresevere level of disability (i.e., considerable difficulties orassistance required in performing daily activities) (21–24).At each HAQ administration, a patient was assigned to 1 of

these 3 disability states. Patients could show deteriorationor improvement in physical functional disability overtheir followup periods.

For modeling this type of staged data, multistate modelsbased on Markov processes provided a natural framework,because interest lay in the course of physical functionaldisability over time and, in addition, patients were onlyunder intermittent observations. This allowed estimationof rates of transitions between the 3 states of functionaldisability and easily incorporated the effects of covariates(both time-independent and time-dependent) on transitionrates. Here, correlation among the states of a patient at thedifferent HAQ assessment visits was directly modeledthrough the Markov assumption that the future evolutionof the patient’s disability process depended only on his/her current state and not on his/her previous history.

An alternative approach that does not require categoriz-ing the HAQ scores is based on random-effects models forlongitudinal data. Here correlation is induced through therandom effects. We preferred the multistate approach, be-cause it allowed us to more directly address our objectives.In addition, distributional assumptions concerning theHAQ score were avoided.

Initially, a multistate Markov model that allowed for-ward and backward transitions from each of the transientdisability states was fit. However, the simplified modelthat eliminated the direct transitions from state 1 to state 3and from state 3 to state 1 was sufficient to characterize ourdata (see Results). Therefore, results reported in this studyare obtained using this “simplified” model (Figure 1).

We assumed that each baseline transition rate remainedconstant throughout the followup period. We further as-sumed that the times of the HAQ assessments were non-informative for the disability process. Note that samplingschemes in which patients are assessed at regular intervalsor assessed at randomly sampled times, or in which thedoctor monitoring the patient’s progress chooses the nextassessment time for the patient depending on the state thepatient is in at the current HAQ assessment visit are allnoninformative. A sampling scheme is informative when apatient who feels unwell and/or whose symptoms suggestthat the disability process is advancing may “self-select” tohave a HAQ assessment visit (25).

We examined the separate (univariate) and joint (multi-variate) effects of select demographic and clinical vari-ables on the transition rates. These covariates were incor-

Figure 1. Reversible multistate Markovian model for observedtransitions between physical functional disability states for 341patients with psoriatic arthritis.

Longitudinal Analysis of Physical Functional Disability in PsA 405

porated into the models through the proportional hazardsassumption. The variables included were sex, age, dura-tion of PsA, psoriasis severity as measured by the PASI,the number of clinically deformed or damaged joints, andthe number of actively inflamed joints updated at eachHAQ visit. Preliminary model investigations showed thatit was adequate to make the simplifying assumption thateach variable had a common effect on all forward transitionsand another common effect on all backward transitions. De-scriptive information on the patients was also provided.

RESULTS

Between June 1993 and June 2003, 395 clinic patients hadcompleted at least 1 HAQ. Of these, 341 patients (86.3%)had completed 2 or more HAQs and comprised the studygroup for our analysis. Demographic and clinical featuresof these 341 patients at their first HAQ administration areshown in Table 1. Briefly, the majority of the patients weremale (59%). The mean age and mean disease duration atthe first HAQ administration were 45.9 years and 10.6years, respectively. PASI scores ranged from 0 to 57.4,with a mean of 7.1. The mean number of actively inflamedand clinically deformed joints (8.3 and 6, respectively)reflected moderate disease activity and disease severity.The majority of patients (62%) had polyarthritis, with orwithout a spondylarthritis, whereas 41% demonstratedevidence of spondylarthritis. With 1 exception, there wereno significant differences in demographic and clinical fea-tures between the 341 patients with 2 or more HAQs andthe 54 patients with 1 HAQ only. At their first HAQ ad-ministration, patients with 1 HAQ had, on average, longerdisease durations (mean duration of PsA 13.9 years).

The mean number of HAQs administered per patientwas 5 (range 2–11), and the mean � SD length of followupwith the HAQ was 5.2 � 3.04 years. The mean � SD

duration between HAQ administrations was 1.29 � 0.70years. At the first HAQ assessment, the mean � SD HAQscore was 0.69 � 0.67, reflecting moderate disability. Onehundred fifty-seven patients (46%) had a HAQ score ofless than 0.5 and thus were assigned an initial disabilitystate of 1, 134 patients (39%) had a score between 0.5 and1.5 (inclusive) and were assigned to disability state 2, andthe remaining 50 patients (15%) had a score greater than1.5 and were assigned to disability state 3.

Table 2 summarizes the number and type of transitionsin disability states that were observed over the followupperiod. Note that not all patients were observed to experi-ence a transition. Of the 341 patients, 95 (28%) were instate 1 (no disability) throughout the entire followup pe-riod, while 42 (12%) and 20 (6%) remained only in state 2(moderate disability) or state 3 (severe disability), respec-tively. One hundred eighty-four of the 341 patients wereobserved to experience a transition in disability state.Ninety-one patients (26.7%) encountered a single transi-tion to either a lower or higher disability state. Ninety-three patients (27.3%) experienced 2 or more observedtransitions, with 1 patient moving progressively to a lowerstate and 92 fluctuating between higher and lower states ofdisability. The mean � SD changes in HAQ score forpatients who moved from a higher to a lower disabilitystate in consecutive visits (i.e., improved) and for thosewho moved from a lower state to a higher state (i.e., dete-riorated) were –0.57 � 0.36 and 0.55 � 0.32, respectively.The mean � SD change in HAQ score for patients whowere not observed to change disability state in consecutivevisits was –0.002 � 0.215.

For the vast majority of patients, the observed transi-tions in disability occurred either between states 1 and 2 orbetween states 2 and 3. Relatively few individuals wereobserved moving directly to and from states 1 and 3. Fig-ure 1 characterizes the disability process of patients in this

Table 1. Demographic and clinical features of 341 PsA patients at their first HAQassessment*

Variable Value Range

No. (%) men 201 (58.9)No. (%) women 140 (41.1)Mean � SD age, years 45.9 � 12.4 17.7–93.6Mean � SD duration of PsA, years 10.6 � 8.4 0.2–60.6Mean � SD number of active joints 8.3 � 9.3 0–55Mean � SD number of joint effusions 3.0 � 4.5 0–30Mean � SD number of deformed joints 6.0 � 11 0–59Mean � SD PASI score 7.1 � 9.7 0–57.4Arthritis pattern, no. (%)

Distal 9 (2.6)Oligoarthritis 53 (15.5)Polyarthritis 121 (35.5)Back alone 14 (4.1)Back � distal 7 (2.1)Back � oligoarthritis 27 (7.9)Back � polyarthritis 90 (26.4)Remission 15 (4.4)Missing 5 (1.5)

* PsA � psoriatic arthritis; HAQ � Health Assessment Questionnaire; PASI � Psoriasis Area and SeverityIndex.

406 Husted et al

study and assumes that an observed transition from state 1to state 3 implies passage through state 2 (and vice versa),even if the time spent in state 2 is brief and unobserved.

Our estimated multistate model, with no covariates, pro-vided estimates of 5.50, 2.26, and 2.61 years for the meannumber of years in each of the 3 disability states, respec-tively. Their respective 95% confidence intervals (95%CIs) were 4.61–6.79, 1.96–2.68, and 2.05–3.58. As ex-pected, Table 2 shows that observed transitions in disabil-ity state were more frequent among patients in state 2 andstate 3 at their first HAQ assessment (69% and 60%, re-spectively) compared with those in state 1 (39.5%).

We examined the univariate effects of age, sex, durationof PsA, psoriasis severity as measured by the PASI, thenumber of clinically deformed joints, and the number ofactively inflamed joints on transition rates in our model.The following results were obtained. The older a patientwas at the time of the HAQ assessment, the slower thepatient was to improve if he or she was currently in eitherstate 2 (moderate disability) or state 3 (severe disability).The transition rate for moving from a higher state to alower one is reduced by 8.5% (95% CI 3.1–13.5%) for apatient 5 years older than another patient, all else beingthe same. Male patients appeared to have a slower rate ofdecline in disability than female patients. Patients withduration of PsA less than 2 years were found to have morefrequent transitions to different states (either to better orworse states). Patients who had PsA for 2–5 years andmore than 5 years had a reduction in transition rates of56–70% compared with those patients with PsA of lessthan 2 years’ duration. There was no evidence to suggestan association between PASI scores and the transition

rates (P � 0.08 and P � 0.79 for moving from a better toworse state, and for moving from a worse to better state,respectively). Finally, patients with a higher number ofclinically deformed joints had, on average, a lower transi-tion rate for improving (relative risk [RR] per joint increase0.98; 95% CI 0.96–0.99), while those patients with ahigher number of actively inflamed joints were quicker toshow deterioration (RR per joint increase 1.04; 95% CI1.02–1.07). When all of these variables (with the exceptionof PASI) were adjusted for in a multistate Markov model,the results shown in Table 3 were obtained and are similarto those obtained univariately.

Finally, we investigated the impact of excluding the 78patients with only 2 HAQ assessments (for whom a fluc-tuating course could not be observed). The results of thisreanalysis were consistent with the reported results. Wealso examined the impact of redefining disability statesinto 5 categories: 0–0.49 (state 1), 0.5–0.99 (state 2), 1.0–1.5 (state 3), 1.51–1.99 (state 4), and 2–3 (state 5). For thisanalysis, we used the total group of patients and found thatthe reported findings did not materially change, with asingle exception. Men were now found to move morerapidly than women back and forth between the newlydefined states 2 and 3 (i.e., within the moderate disabilitystate defined earlier for the 3-state model). However, menmoved less frequently than women from state 1 to state 2and between state 3 and state 4.

DISCUSSION

This study is the first to examine the longitudinal course ofphysical functional disability in PsA. Although several

Table 2. Number and type of observed transitions between disability states for 341 PsApatients*

Number and type

Disability state at time of firstHAQ assessment

State 1(n � 157)

State 2(n � 134)

State 3(n � 50)

No transitions (n � 157) 95 42 20One transition (n � 91)

DeteriorationState 1 3 2 25 0 0State 1 3 3 1 0 0State 2 3 3 0 10 0

ImprovementState 2 3 1 0 36 0State 3 3 1 0 0 3State 3 3 2 0 0 16

Two or more transitions (n � 93)Steady observed deterioration

State 1 3 2 3 3 0 0 0Steady observed improvement

State 3 3 2 3 1 0 0 1Fluctuating course, both deterioration and improvement

State 1 N 2 36 29 0State 2 N 3 0 14 8State 1 N 2, 1 N 3, 2 N 3 0 3 2

* PsA � psoriatic arthritis; HAQ � Health Assessment Questionnaire.

Longitudinal Analysis of Physical Functional Disability in PsA 407

reports have indicated that patients with PsA experiencereduced physical functioning (9–11), all of these studieshave used a cross-sectional design. Because physical func-tional disability is known to fluctuate over time (26,27),cross-sectional measures of physical function may providea misleading picture of the burden of disability.

Our results indicated that although there was patientvariability in the course of physical functional disability, 3longitudinal patterns could be observed. The first reflecteda stable state of disability throughout the study period,with 28% of the patients experiencing no disability overthe study duration, and 12% and 6% experiencing mod-erate or severe disability, respectively. The second patternwas one of either steady improvement or deterioration andoccurred in �27% of patients. With 1 exception, theseindividuals were observed to undergo a single transitiononly. The third pattern was characterized by multiple tran-sitions and fluctuating states of disability over the studyperiod and was also observed in 27% of patients.

Figure 1 illustrates the multistate Markov model used tocharacterize the disability process in PsA. This modelallows for direct transitions only between no disabilityand moderate disability and between moderate disabilityand severe disability. Observed transitions from no dis-ability to severe disability (and vice versa) involved pas-sage through moderate disability. The model also providedestimates of time spent in each of the 3 disability states.The mean number of years spent in either the state ofmoderate or severe disability (2.26 and 2.61 years, respec-tively) was lower than that spent in the state of no disabil-ity (5.50 years), reflecting the fact that observed transitionsin disability occurred more frequently in patients whoentered the study with moderate or severe disability.

In the multivariate analysis, sex, age, disease duration,number of actively inflamed joints, and number of de-formed joints significantly influenced transition rates. Inthe literature on disability (27–29), female sex has beenconsistently associated with higher levels of physical dis-ability. Similar to these past studies, we found that beingfemale increased the likelihood for progression of disabil-

ity. In terms of age, increasing age was found to decreasethe likelihood of improvement among patients with mod-erate or severe disability. Older patients were thereforemore likely to experience persistent disability than wereyounger patients. Sherrer et al (30) found that age was themost powerful single predictor of subsequent disability inRA but showed that the predictive power of age is partiallydependent on its interrelationships with other factors re-lated to long-term disability, namely disease duration, co-morbid conditions, and frailty. In this study, the effect ofincreasing age remained after adjusting for disease dura-tion. With respect to disease duration, our results are con-sistent with those from longitudinal studies in RA (26,31),reporting more variability in levels of disability during theearly course of RA compared with that in the later courseof disease. There are several possible explanations for thisobservation. Wiles et al (26) argue that in early RA, func-tional disability may fluctuate considerably due to spon-taneous changes in disease activity, variability in timingand response to disease-modifying drugs, coping strate-gies, and adaptation to RA. In contrast, joint damage startsto accumulate in later disease, leading to an increase inpersistent disability. It has also been suggested that theefficacy of treatment may be reduced over time. This, too,may result in an increase of enduring disability in laterdisease. Alternatively, patients with longer disease dura-tion may represent a group of patients who failed to re-spond to past treatment and consequently experiencedenduring disability. Interestingly, in the multivariatemodel, the effect of disease duration remained after adjustingfor both number of actively inflamed joints and number ofdeformed joints. Because we did not include either treatmentresponse or measures of coping and illness adaptation in ourmodeling approach, these variables might partially explainthe observed relationship between disease duration and tran-sition rates. As expected, a higher number of actively in-flamed joints was associated with subsequent deteriorationin disability, and the number of deformed joints reduced thelikelihood of improvement in functional disability state.

Some methodologic issues related to this study need to

Table 3. Results from the multivariate analysis that identified predictors of transitionsbetween disability states*

Variable

Transitions

1 3 2 2 3 3 2 3 1 3 3 2Relative risk (95% CI) Relative risk (95% CI)

SexMale 0.54 (0.38–0.76) 0.92 (0.66–1.28)Female 1.00 1.00

Age 1.01 (0.99–1.03) 0.99 (0.97–1.00)Duration of PsA, years

�2 1.00 1.002–5 0.42 (0.16–1.09) 0.33 (0.14–0.77)�5 0.33 (0.14–0.76) 0.44 (0.21–0.90)

No. of clinically deformed joints 1.00 (0.99–1.01) 0.98 (0.97–0.99)No. of actively inflamed joints 1.03 (1.01–1.06) 0.99 (0.97–1.01)�2 log-likelihood 1,716.885

* 95% CI � 95% confidence interval; PsA � psoriatic arthritis.

408 Husted et al

be addressed in future research. To observe a fluctuatingcourse in disability over time, all patients ideally shouldhave at least 3 HAQ assessments. This was not the case for78 patients who had only 2 HAQ assessments, althoughthe findings did not materially change when these patientswere excluded from the analysis. Ongoing followup of thissample will allow us not only to assess the robustness ofour characterization of physical disability in PsA, but alsoto investigate additional factors that predict changes in thelevel of disability, such as medication history, comorbidconditions, and radiologically detected joint damage.

In summary, although 28% of patients appeared resis-tant to becoming disabled over the study duration, theremaining patients were observed either to have enduringdisability or to move between disability states. Futureresearch should identify factors in addition to age, sex,disease duration, and number of actively inflamed anddeformed joints that predict changes in disability over thecourse of illness.

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