Explanation

The care density was first proposed by Pollack et al. (2013). It is a measure that is supposed to measure how closely the “care team” of a patient (the providers / medical doctors he is in contact with) are connected. As an indirect measure of how closely two providers work together, the number of patients shared by the providers is used. Therefore, in order to calculate the care density, the only information we need is which patient visited which provider. This information can be considered a “patient-sharing network” and has been used in multiple studies already (DuGoff et al. 2018, Landon et al. 2012).

Formally, the care density is defined as:

$$C_p = \frac{\sum_{i = 1}^m w_{p, j}}{n_p (n_p - 1)/2},$$

with n_p being the number of providers a patient has visited, m defined as the number of all possible combinations of length two and w_p, j being the number of patients that a pair of provider is sharing.

Example

Suppose we have the following data about 5 patients (denoted by numbers) and the 3 providers they visited (denoted by uppercase letters):

library(CareDensity)
library(igraph)
library(data.table)
library(MatrixExtra)

set.seed(3431)

# some arbitrary patient-provider contact data
data <- data.frame(PatID=c("1", "1", "1", "2", "2", "3", "3", "4", "5"),
                   ArztID=c("A", "C", "D", "A", "D", "A", "D", "D", "C"))
data
#>   PatID ArztID
#> 1     1      A
#> 2     1      C
#> 3     1      D
#> 4     2      A
#> 5     2      D
#> 6     3      A
#> 7     3      D
#> 8     4      D
#> 9     5      C

Since there are only connections between patients and providers and no connections between provider and providers or patients and patients, this can be considered a bipartite graph. We can visualize this graph using the igraph package:

# create graph
g <- graph_from_data_frame(data, directed=FALSE)

# add type
V(g)$type <- bipartite_mapping(g)$type

# change some things for better plots
V(g)$color <- ifelse(V(g)$type, "salmon", "lightblue")
V(g)$shape <- ifelse(V(g)$type, "square", "circle")
E(g)$color <- "lightgray"

plot(g, vertex.label.cex=0.8, vertex.label.color="black")

Consider patient number 1 in this graph. We can see that this patient visited 3 different providers (A, B and C). Therefore, n_p is equal to 3 for this patient. But how about the weights? To gain a better understanding, we can project this graph into a provider-centric network, where only providers are included:

# project to provider mode
mat <- project_to_one_mode(g, mode="cols")

# make it an igraph object again
g_provider <- graph_from_adjacency_matrix(mat, mode="upper", weighted=TRUE, diag=FALSE)

# plot it
plot(g_provider, edge.label=E(g_provider)$weight, vertex.shape="square",
     edge.color="black", vertex.color="salmon",
     vertex.label.cex=0.8)

In this graph we can see exactly which provider shared how many patients. So for patient 1 the care density would be:

$$C_1 = \frac{1 + 1 + 3}{3 (3 - 1)/2} \approx 1.6667.$$

While this is easy to calculate for a single patient in a small network, it is unfeasible to do this by hand in a large network and for multiple patients. The care_density() function can be used to automatically perform the step taken by hand above. All we need to do is supply the original data to it:

care_density(data)
#>   PatID sum_weights n care_density
#> 1     1           5 3     1.666667
#> 2     2           3 2     3.000000
#> 3     3           3 2     3.000000
#> 4     4          NA 1           NA
#> 5     5          NA 1           NA

Here we can see that it gets the same result for patient 1 and also outputs the care density for all other patients. Since patient 4 and 5 have only visited one provider, the care density is undefined for them and is thus denoted by NA.

Explanation

The care density as defined by Pollack et al. (2014) has recently been criticed by Engels et al. (2024). Their main point are that:

• 1.) The care density assumes that all relationships with the same amount of patients shared contribute equally to care coordination. It therefore does not consider how many patients a single provider has.
• 2.) The kind of relationship is also not considered in the classic care density statistic. For example, relationships between psychiatrists and general providers may be more or less important than general provider - general provider relationships.
• 3.) The importance of specific relationships may vary for different outcomes and or different patient population, which is therefore also not considered in the classic care density measure or Pollack et al. (2024).

Engels et al. (2024) instead propose a modified version of the care density, which they call fragmented care density. The main idea is to stratify the sum term of the classic care density into multiple provider-relationship specific sums. $$C_p = \sum_{j = 1}^{k} \frac{s_j}{n_p(n_p - 1) /2},$$

with k being defined as:

$$k = {l \choose 2} + l,$$

where l is the number of provider types. For example, if we have three different provider types (psychiatrist, general provider, other), we would have 6 different relationships. It is easy to see that this formulation is equivalent to the one given by Pollack et al. (2013). The new proposal is to weight the different connection types differently. The fragmented care density as proposed by Engels et al. (2024) is then given by:

$$FC_p = \sum_{j = 1}^{k} w_j \frac{s_j}{n_p(n_p - 1) /2}.$$

where w_j are some connection specific weights. Engels et al. (2024) propose to use regression models to estimate these weights. This is explained in a little more detail below.

Example

Consider again the example dataset defined above. Suppose, however, that each provider is known to be either a general practitioner or a psychiatrist. The following data.frame defines the type of each provider in our example:

d_type <- data.frame(ID=c("A", "C", "D"),
                     Type=c("GP", "GP", "Psychiatrist"))
d_type
#>   ID         Type
#> 1  A           GP
#> 2  C           GP
#> 3  D Psychiatrist

Since there are two different kinds of providers, there are 3 possible provider connections:

• GP - GP
• GP - Psychiatrist
• Psychiatrist - Psychiatrist

We can safely omit the “Psychiatrist - GP” connection type, because we are always dealing with undirected connections, which means that the order of appearance does not make a difference. Now we can use the fragmented_care_density() function as follows:

fragmented_care_density(data, weights=NULL, type=d_type, by_connection=TRUE)
#>   PatID        connection sum_weights n care_density
#> 1     1           GP - GP           1 3    0.3333333
#> 2     1 Psychiatrist - GP           4 3    1.3333333
#> 3     2 Psychiatrist - GP           3 2    3.0000000
#> 4     3 Psychiatrist - GP           3 2    3.0000000
#> 5     4              <NA>          NA 1           NA
#> 6     5              <NA>          NA 1           NA

By setting by_connection to TRUE we are telling the function to not directly calculate the fragmented care density for us, but to instead return the sum of the weights and care densities per connection type and patient. From this dataset we can see that only patient 1 has two different kinds of provider connections among his network. If we sum up the care densities of each patient over all his connection types, we get the classic care density.

To calculate the actual fragmented care density for each patient, we need to supply weights for each connection type. We can do this by constructing a data.frame that looks like this:

d_weights <- data.frame(from=c("GP", "GP", "Psychiatrist"),
                        to=c("GP", "Psychiatrist", "Psychiatrist"),
                        weight=c(1, 1, 1))
d_weights
#>           from           to weight
#> 1           GP           GP      1
#> 2           GP Psychiatrist      1
#> 3 Psychiatrist Psychiatrist      1

Here, we set all weights to 1. Therefore, if we now call the fragmented_care_density() function again (this time without using the by_connection argument):

fragmented_care_density(data, weights=d_weights, type=d_type)
#>   PatID fragmented_care_density
#> 1     1                1.666667
#> 2     2                3.000000
#> 3     3                3.000000
#> 4     4                      NA
#> 5     5                      NA

we get the same results as we did before when using the simple care_density() function. However, if we use different weights for each connection type:

d_weights <- data.frame(from=c("GP", "GP", "Psychiatrist"),
                        to=c("GP", "Psychiatrist", "Psychiatrist"),
                        weight=c(1.1, 0.5, 1.3))
d_weights
#>           from           to weight
#> 1           GP           GP    1.1
#> 2           GP Psychiatrist    0.5
#> 3 Psychiatrist Psychiatrist    1.3

and call the function again:

fragmented_care_density(data, weights=d_weights, type=d_type)
#>   PatID fragmented_care_density
#> 1     1                1.033333
#> 2     2                1.500000
#> 3     3                1.500000
#> 4     4                      NA
#> 5     5                      NA

we obtain different results. Note that this never matters for patients 4 and 5, because they only have one provider connection, resulting in NA values no matter the weights.

Estimating Connection-Specific Weights

In the example above, we used arbitrary weights to showcase the functionality of the fragmented_care_density() function. In reality we would like to use sensible weights instead. Engels et al. (2024) recommend using regression models to obtain the relevant weights. In their article, they use a logistic regression model to predict a binary outcome, given the presence of a connection type and some covariates. We will illustrate this approach here without the use of other covariates.

Suppose in addition to data (patient-provider relationships) and type (the provider types) we also have some patient level information on a binary outcome of interest Y. Suppose this is that data:

d_outcome <- data.frame(PatID=c("1", "2", "3", "4", "5"),
                        Y=c(0, 0, 1, 1, 0))
d_outcome
#>   PatID Y
#> 1     1 0
#> 2     2 0
#> 3     3 1
#> 4     4 1
#> 5     5 0

First, we again obtain data about the connection sums (s_j) per connection type:

d_consum <- fragmented_care_density(data, weights=NULL, type=d_type, by_connection=TRUE)

We then binarize these weights to 0 / 1. If at least one connection is present, it is considered a 1, otherwise it is a 0:

d_consum$sum_weights <- fifelse(d_consum$sum_weights > 0, 1, 0)

Next, we transform this information into the long-format using the dcast() function of the data.table package:

d_consum <- dcast(as.data.table(d_consum), PatID ~ connection,
                  value.var="sum_weights", fill=0)
d_consum$`NA` <- NULL

Afterwards we can merge the outcome data to this dataset:

d_outcome <- merge(d_consum, d_outcome, by="PatID")

Now we can finally fit the regression model, using the outcome as the dependent variable and the connection-type dummies as independent variables:

mod <- glm(Y ~ `GP - GP` + `Psychiatrist - GP`, data=d_outcome, family="binomial")
summary(mod)
#> 
#> Call:
#> glm(formula = Y ~ `GP - GP` + `Psychiatrist - GP`, family = "binomial", 
#>     data = d_outcome)
#> 
#> Coefficients:
#>                       Estimate Std. Error z value Pr(>|z|)
#> (Intercept)         -1.256e-16  1.414e+00   0.000    1.000
#> `GP - GP`           -1.857e+01  6.523e+03  -0.003    0.998
#> `Psychiatrist - GP`  0.000e+00  2.000e+00   0.000    1.000
#> 
#> (Dispersion parameter for binomial family taken to be 1)
#> 
#>     Null deviance: 6.7301  on 4  degrees of freedom
#> Residual deviance: 5.5452  on 2  degrees of freedom
#> AIC: 11.545
#> 
#> Number of Fisher Scoring iterations: 17

The estimated coefficients associated with each connection type may now be used as weights. Note that in this example the weights don’t make any sense due to (1) the very low sample size, (2) the very small network and (3) because all data is completely made up with no underlying true relationships. The general procedure, however, should work with real data.