Table 4.

Final multivariable logistic regression model^a

Variable	Coefficient estimate	MLR
		Standard error	Odds ratio (95% CI)	Wald's chi‐square test	P
Intercept	−1.0864	0.3299	0.337 (0.177–0.644)	10.8445	0.0010
Mean no. of capillaries/mm in middle finger of dominant hand	−0.1770	0.0670	0.838 (0.735–0.955)	6.9801	0.0082
No. of DUs at enrollment
1	0.7460	0.3307	2.109 (1.103–4.032)	5.0878	0.0241
2	1.1696	0.3889	3.221 (1.503–6.902)	9.0458	0.0026
≥3	1.8181	0.3672	6.160 (2.999–12.653)	24.5082	<0.0001
Critical digital ischemia present at enrollment	1.1613	0.4649	3.194 (1.284–7.945)	6.2388	0.0125

^aThe final prognostic model used 3 variables to predict the occurrence of digital ulcers (DUs) within 6 months: the mean number of capillaries/mm in the middle finger of the dominant hand, the number of DUs at enrollment (categorized as 0, 1, 2, or ≥3), and the presence/absence of critical digital ischemia (defined as prolonged, severe, persistent reduction in digital tissue perfusion without rewarming) at enrollment. The multivariable logistic regression (MLR) coefficient estimates indicate that the risk of developing a DU within 6 months increases in patients with critical digital ischemia at enrollment, in patients with a greater number of DUs, and in patients with a lower number of capillaries/mm in the middle finger of the dominant hand. No variable among the demographic, SSc clinical, or other clinical characteristic bundle was retained. Receiver operating characteristic curve analysis showed an area under the curve of 0.738 (95% confidence interval [95% CI] 0.681–0.795). The Hosmer‐Lemeshow goodness‐of‐fit test yielded the following values: χ² = 5.0602, 8 df, P = 0.751. The MLR equation, based on the estimates shown in the table, was as follows:

$$\text{Probability} \text{of} \text{new} \text{DUs} \text{within} 6 \text{months} = \text{exp}\left(\text{linear} \text{predictor}\right)/\left[1 + \text{exp}\left(\text{linear} \text{predictor}\right)\right]$$

where the linear predictor is −1.0864 (intercept) −0.1770 multiplied by the mean number of capillaries per mm in the middle finger of the dominant hand, plus either 0.7460 for the presence of 1 DU, 1.1696 for 2 DUs, or 1.8181 for ≥3 DUs at enrollment, plus 1.1613 for the presence of critical digital ischemia at enrollment. Thus, a patient with 5 capillaries/mm in the middle finger of the dominant hand, plus 2 DUs at enrollment, plus critical digital ischemia at enrollment has a 59% probability of developing new DUs within 6 months.