An argument for reporting data standardization procedures in multi-site predictive modeling: case study on the impact of LOINC standardization on model performance

Abstract

Objectives

We aimed to gain a better understanding of how standardization of laboratory data can impact predictive model performance in multi-site datasets. We hypothesized that standardizing local laboratory codes to logical observation identifiers names and codes (LOINC) would produce predictive models that significantly outperform those learned utilizing local laboratory codes.

Materials and Methods

We predicted 30-day hospital readmission for a set of heart failure-specific visits to 13 hospitals from 2008 to 2012. Laboratory test results were extracted and then manually cleaned and mapped to LOINC. We extracted features to summarize laboratory data for each patient and used a training dataset (2008–2011) to learn models using a variety of feature selection techniques and classifiers. We evaluated our hypothesis by comparing model performance on an independent test dataset (2012).

Results

Models that utilized LOINC performed significantly better than models that utilized local laboratory test codes, regardless of the feature selection technique and classifier approach used.

Discussion and Conclusion

We quantitatively demonstrated the positive impact of standardizing multi-site laboratory data to LOINC prior to use in predictive models. We used our findings to argue for the need for detailed reporting of data standardization procedures in predictive modeling, especially in studies leveraging multi-site datasets extracted from electronic health records.

INTRODUCTION

The growing repository of available healthcare data has motivated the healthcare community to improve medical decision-making by integrating knowledge learned from data-driven analyses.^1,2 Often, these analyses are geared toward enhancing clinical decision support (CDS) systems with models that predict events of clinical relevance, such as disease risk or progression.² Laboratory data are particularly valuable information in predictive modeling as they can provide insight about a patient’s current and potential future clinical state. Unfortunately, the secondary use of laboratory data poses challenges due to the lack of enforced standardization.³ Currently, the only available standard for lab tests is the logical observation identifiers names and codes (LOINC), which provides a universal set of structured codes to identify laboratory and clinical observations.^4,5 We have noticed in the literature, however, that most predictive modeling studies utilizing clinical laboratory data provide little to no information on the standardization processes used.

As an illustrative example of the lack of reporting in the literature, we considered readmission risk prediction models, which have grown increasingly popular since the introduction of financial penalties for excess readmissions by the Centers for Medicare and Medicaid Services (CMS).⁶ A number of studies on predicting readmission risk have utilized laboratory data^7–24; however, most multi-site readmission prediction models using laboratory information provide limited details on the data standardization procedures used across sites.^{8,9,11–13,15,16,23,24} In particular, we found only 1 study that included any traceable record of standardizing to LOINC.¹² Failing to report standardization procedures makes it challenging to accurately reproduce these multi-site predictive models and presents potential methodological issues in the modeling approach. For example, if a multi-site study failed to standardize laboratory test names across sites, it would result in incorrectly treating clinically comparable laboratory tests from different sites as unique tests in the model. This could result in poor overall model performance in addition to potentially mitigating the predictive power of laboratory data. This risk is especially high for data-driven modeling approaches, which are gaining popularity in the healthcare domain.²⁵ The potential impact of standardizing laboratory data on prediction performance in multi-site datasets, however, has been largely ignored and under reported.

OBJECTIVES

In this study, we aimed to gain a better understanding of how the standardization of laboratory data can impact predictive model performance. We specifically focused on understanding how standardizing local laboratory test codes to LOINC impacts predictive model performance in multi-site datasets. We hypothesized that standardizing local laboratory codes to LOINC would produce predictive models that significantly outperform those learned utilizing local laboratory codes. To test our hypothesis, we performed a case study using 30-day readmission risk predictive models for adult heart failure patients, as this population is currently subject to financial penalties by the CMS.⁶ Findings from our study were used to construct an argument for the importance of reporting data standardization procedures in multi-site predictive modeling studies. The main contributions of this work included: (1) empirical evidence to support the need for data standardization in predictive modeling using multi-site datasets and (2) suggested recommendations for reporting laboratory data standardization.

METHODS

We extracted laboratory test results for adult heart failure patient visits from a large, multi-hospital health system. We then cleaned and standardized test results and mapped local laboratory test codes to LOINC. We constructed a set of features, and then learned several models to predict risk of 30-day hospital readmission using a variety of feature selection and modeling techniques. We compared the performance of models learned using local laboratory test codes to the same models learned using LOINC. These processes are described in detail in the following sub-sections. This study was reviewed and approved by the institutional review board (IRB) at the University of Pittsburgh (PRO18040108).

Dataset

We utilized an IRB certified honest broker to retrieve all electronic health records (EHRs) for in-patient visits to 13 individual hospitals within the University of Pittsburgh Medical Center (UPMC) Health System from 2008 to 2012. Heart failure-specific visits were identified using primary discharge ICD-9 codes [428 family (428.XX), 402.01, 402.11, 402.91, 404.01, 404.03, 404.11, 404.13, 404.91, 404.93]. Visits with in-hospital deaths and any visit without at least 1 valid laboratory test value available were excluded. If a patient returned to any UPMC hospital within 30 days following discharge from a visit, then the visit was classified as “Readmitted” (R); otherwise the visit was classified as “Not Readmitted” (NR). All visit information was then deidentified by the honest broker and provided to the research team for analysis. Laboratory test results from each visit were manually cleaned and standardized, and then were flagged as normal/abnormal (a detailed report of cleaning and standardization procedures is available in the Supplementary Material). Only data collected prior to discharge from the visit were used to predict whether the visit would result in a 30-day readmission, that is, only data from the initial visit were included in the prediction model.

Mapping to LOINC

As part of an ongoing effort to convert to LOINC, 1 UPMC hospital had previously mapped 456 of the most commonly ordered local laboratory test codes to LOINC. At the time of this study, this partial mapping was the only available mapping to LOINC across all 13 hospitals. The mapping process was completed manually by 3 coders from the Laboratory Information System (LIS) division who had more than 20 years of clinical laboratory experience and medical technologist certifications from the American Society for Clinical Pathology. Two coders independently mapped local laboratory codes to LOINC and discussed discrepancies. A third coder (T.G.) oversaw the process and reviewed discrepancies if the two coders could not come to an agreement. A supervisor of the UPMC core laboratory vetted the resulting list of LOINC assignments as a final technical review. Unfortunately, this list was not originally generated for research use, therefore the intercoder reliability was not captured, initial false positive mappings were not racked, and no formal validation of the mapping process was able to be performed.

UPMC hospitals’ local laboratory test codes consist of a descriptive code for the test and a hospital ID tag indicating the source hospital (e.g, code “K14” represents a serum potassium test for hospital with ID 14). By removing the hospital ID tags, we were able to use the list of 456 mapped codes from a single hospital to map local laboratory codes to LOINC for all 13 hospitals. This process yielded 2 datasets for analysis: (1) a “non-standardized” dataset where tests were identified via the local laboratory codes (ie, no mapping of laboratory codes was performed) and (2) a “standardized” dataset where tests were identified via LOINC. An example of the mapping process is illustrated in Figure 1. As only a partial LOINC mapping was available, we discarded any tests that could not be mapped to LOINC from both the “non-standardized” and “standardized” datasets. This was done to ensure that we compared model performance across the same set of laboratory tests to get an unbiased estimate of the effect of standardizing laboratory codes to LOINC.

Figure 1.

Example of LOINC mapping for potassium laboratory tests. A manual mapping from 1 hospital (left) was extended to map local laboratory test codes from 13 hospitals to LOINC. After mapping, we had an “non-standardized” dataset, where laboratory tests were identified via the unmapped, local laboratory test codes and a “standardized” dataset, where laboratory tests were identified via a LOINC code.

Feature construction

Due to the asynchronous, time-series nature of laboratory data, we defined a fixed set of features to summarize test results for each patient visit. The features are listed in Table 1. Many of these features were part of a laboratory data feature set originally described by Hauskrecht et al.,²⁶ but we also derived some new features. In Table 1, we have identified the Hauskrecht et al.²⁶ features with superscript ‘H’s. To summarize the results for all laboratory tests that occurred during a patient visit, we defined 3 features (Table 1, column 1): (1) the average number of test results received per day (defined as number of tests divided by length of stay), (2) the percentage of most recent test results that were flagged as abnormal, that is, the percentage of abnormal results when considering only the most recently recorded result from each test, and (3) the percentage of all test results that were flagged as abnormal. For each categorical lab test, results were summarized using the results from the 2 most recent tests, the result from the first test, and the baseline result across all tests, which was defined as the mode of all test results excluding the most recent test (Table 1, column 2). For each continuous lab test, results were summarized using the percentage of all test results that were flagged as abnormal, the results from the 2 most recent tests, the result from the first test, the baseline result across all tests (defined as the mean of all test results excluding the most recent test), the nadir (min) and apex (max) results from all tests, and several features aimed to summarize result trends over time, such as the difference, percent change, and slope between the 2 most recent test results (Table 1, column 3). To reduce the amount of missing data generated in constructing the feature set, some features were only constructed for a given test if the median number of results per patient for that test was greater than 1 or 2. For example, for a categorical test for which most patients only have 1 test result, we would only use the most recent test result as a feature. Features were constructed for both the “non-standardized” and “standardized” datasets. All numeric constructed features were discretized using the minimum description length criterion discretization method.²⁷

Table 1.

Features constructed to summarize laboratory test results each patient visit

Included features		Summary of results for
		All lab tests	Each categorical lab test	Each continuous lab test
Average # of tests per day (# tests/length of stay)		X
% Abnormal tests for most recent testsa		X
% Abnormal testsa		X		X
Flag (normal/abnormal) for most recent test				X
Most recent test result			XH	XH
Second most recent test result (if median test count >1)			XH	XH
First test result (if median test count >2)			XH	X
Baseline result (mean/mode of values prior to most recent) (if median test count>1)			X	XH
Nadir (min) result (if median test count>2)				XH
Apex (max) result (if median test count>2)				XH
Difference between most recent test result and….	Second most recent test result			XH
	First test result			X
	Apex result			XH
	Nadir result			XH
	Baseline result			XH
% change between most recent test result and…	Second most recent test result			XH
	First test result			X
	Apex result			XH
	Nadir result			XH
	Baseline result			XH
Slope between most recent test result and…	Second most recent test result			XH
	First test result			X
	Apex result			XH
	Nadir result			XH
	Baseline result			XH

X: feature was derived for dataset; H: feature was originally described in Hauskrecht et al.²⁶

^aTests with “NA” flags were not included in these computations.

Model learning and evaluation

To learn and validate predictive models, we split each of the “non-standardized” and “standardized” datasets into a training dataset (data from 2008 to 2011) and a test dataset (data from 2012). We used the training datasets to learn models utilizing a variety of popular feature selection techniques and model types. We examined 2 popular strategies for feature selection: (1) correlation-based feature subset (CFS)²⁸ selection which aims to find a set of features that have high correlation with the target class but low intercorrelation with each other, that is, a set of non-redundant, highly informative features and (2) information gain (IG) filter with a threshold greater than 0, which results in selecting features that contain at least some information with respect to the target class. For models, we examined logistic regression, naïve Bayes, and random forest classifiers, which are three popular models within the medical domain. We used the WEKA (Waikato Environment for Knowledge Acquisition) version 3.8²⁹ implementation of all algorithms. We adopted the default algorithm settings provided by WEKA, except for treating missing values as a separate category in our feature selection approaches, which had been previously shown to improve model performance,³⁰ and learning a larger number of trees (500) in the random forest classifier. For each feature selection and classifier pair, we learned a predictive model based on the “non-standardized” and “standardized” datasets. The learned models are summarized in Table 2.

Table 2.

30-Day heart failure readmission model descriptions, evaluations, and comparisons. Prior to feature selection, there were 10,032 and 1881 features from non-standardized dataset (local codes) and standardized dataset (LOINC) respectively.

#	Feature selection	Classifier	Dataset	Number of features	AUC (95% CI)	P-value
1	Information gain	Logistic regression	Non-standardized (Local codes)	1154	0.538 (0.516–0.559)	0.001
2			Standardized (LOINC codes)	388	0.573 (0.551–0.594)
3		Naïve Bayes	Non-standardized (Local codes)	1154	0.560 (0.539–0.582)	5.3e-5
4			Standardized (LOINC codes)	388	0.603 (0.583–0.624)
5		Random forest	Non-standardized (Local codes)	1154	0.590 (0.570–0.612)	0.036
6			Standardized (LOINC codes)	388	0.605 (0.585–0.626)
7	Correlation-based feature selection	Logistic regression	Non-standardized (Local codes)	57	0.566 (0.545–0.587)	2.3e-4
8			Standardized (LOINC codes)	46	0.601 (0.580–0.622)
9		Naïve Bayes	Non-standardized (Local codes)	57	0.571 (0.550–0.592)	8.9e-6
10			Standardized (LOINC codes)	46	0.607 (0.586–0.628)
11		Random forest	Non-standardized (Local codes)	57	0.561 (0.539–0.582)	2.5e-4
12			Standardized (LOINC codes)	46	0.602 (0.581–0.622)

Note: Bolded P-values indicate significant differences in model performance.

We used the respective test datasets to evaluate the learned predictive models. All evaluation metrics were computed using the pROC package³¹ version 1.13.0 in R version 3.4.³² Evaluation metrics for each model included the area under the receiver-operating characteristic curve (AUC) and the 95% confidence interval (CI) computed using 2000 stratified bootstrap replicates (see pROC package documentation for details on bootstrapping approach).³³ DeLong’s 1-sided comparisons³⁴ with Bonferroni multiple-hypotheses correction³⁵ were used to compare AUCs of the models based on the “non-standardized” and “standardized” datasets.

RESULTS

Figure 2 summarizes the coverage of the mapping process and provides a description of the training and test datasets. Table 2 summarizes the models learned to predict 30-day hospital readmission for adult heart failure patients, including the number of features used based on each feature selection technique, the AUC with 95% CI, and the P-values of the model comparisons. Complete lists of features selected by the CFS method for each dataset are provided in Table A1 of the Supplementary Material. As indicated by the bold-faced P-values in Table 2, nearly all models learned on the “standardized” dataset (ie, where tests were identified via LOINC) performed significantly better than models learned on the “non-standardized” dataset (ie, where tests were identified via local laboratory codes).

Figure 2.

LOINC mapping coverage and description of training and test datasets. “R” and “NR” stand for the classification as “Readmitted” or “Not Readmitted”, respectively.

DISCUSSION

We examined the effect of standardizing local laboratory test names to LOINC on predictive model performance in multi-site datasets. More specifically, we evaluated this effect in a case study on predicting 30-day hospital readmissions for a multi-site cohort of adult heart failure patients. To the best of our knowledge, this is the first study to examine this effect. Our results in Table 2 demonstrated that standardizing local laboratory codes to LOINC for multi-site datasets consistently resulted in models that achieved significantly higher predictive performance, regardless of the feature selection technique and classifier approach used. The final AUCs of our models were modest; however, the goal of this study was not to build a high-performing model, but rather to determine whether standardization of laboratory test names to LOINC improved model performance. We noticed significant improvement in performance even with the limited predictive ability of our models, and we believe that higher performing models using additional data would also benefit from standardization of laboratory data. This could lead to better overall predictive models to be used in CDS systems, especially since previous work has shown that standardization of data tends to lead to better outcomes for CDS systems.³⁶ Given the potential impact standardizing laboratory data might have on predictive model performance, we find it alarming that many multi-site predictive modeling studies fail to include details on laboratory data standardization.

The low quality of reporting of prediction model studies in the healthcare domain has been previously identified as an issue, and it presents challenges in reproducing models and assessing the potential bias and usefulness of the models.³⁷ Efforts have been made to develop recommendations for researchers when reporting the development and validation of models, such as the Transparent Reporting of a multivariate prediction model for Individual Prognosis or Diagnosis (TRIPOD) statement.³⁷ The TRIPOD statement is an excellent guideline for transparent model reporting and has been used to describe machine learning modeling approaches,³⁸ but it provides limited consideration for data-driven approaches that utilize multi-site datasets. Specifically, it offers no guidance for reporting data standardization procedures. As our study has demonstrated, the standardization procedures used can have a profound impact on model performance and reproducibility when employing an EHR data-driven approach to prediction. Thus, detailed reporting on standardization procedures seems crucial to critically evaluate such models. These aspects will become an increasingly important part of predictive model reporting as EHR data-driven approaches to prediction gain popularity. Therefore, we argue that current predictive model reporting recommendations should be expanded to consider some of the unique challenges present when modeling with multi-site datasets extracted from EHRs. In particular, we argue for explicit recommendations pertaining to the reporting of data standardization procedures across sites.

Specific attention should be given to developing recommendations for reporting standardization procedures for laboratory data. Although LOINC is the accepted standard for reporting laboratory test names, it is a highly specific coding system and there is no standard procedure for mapping to LOINC. This presents a granularity problem when performing LOINC mapping.^5,39–46 Therefore, wide variation in mapping specificity exists across institutions,^39,47,48 which may pose significant challenges in predictive modeling on multi-site datasets. In a dataset with multiple mapping approaches performed by different institutions, effects on model performance due to varying levels of mapping specificity may be comparable to those observed in our study. We therefore recommend that multi-site studies evaluate and report on any differences in LOINC mapping processes used across sites. When possible, studies should report the level of agreement between LOINC mappings from different institutions.

Several prior studies specifically point out the need for lower resolutions of LOINC (eg, code groups or hierarchical structuring) to promote accurate data sharing and analysis across institutions.^39,43,46,48 This need will become increasingly prevalent as more initiatives are undertaken to create and analyze networks of healthcare data across multiple institutions, such as the National Patient-Centered Clinical Research Network.⁴⁹ The new LOINC Groups project by the Regenstrief Institute aims to address this need by creating sets of clinically similar codes. When completed, LOINC Groups could prove to be an invaluable tool for grouping the LOINC mappings in large multi-institutional datasets in a clinically meaningful way.⁵⁰ As suggested by the findings of our study, these groupings may improve the quality and performance of predictive models learned from these networks of data. Without detailed reporting of the data standardization procedures used, however, it may be challenging to critically appraise and reproduce predictive models learned from these large, multi-institutional datasets. We therefore recommend that as part of laboratory data standardization reporting requirements, future studies should include any LOINC aggregation procedures used. In particular, we suggest that once the LOINC Groups project is completed, it should be recommended as the standard approach for aggregating codes.

Recently, an argument was made against the need for EHR data standardization and harmonization due to advancements in deep learning approaches to modeling, which are capable of achieving high performance when using large sets of messy data.⁵¹ Although our work did not explore deep learning approaches, it is worth discussing the idea as it contradicts our argument for the need for reporting data standardization procedures. The deep learning approach is a promising avenue for achieving high performance models based on raw EHR data, but these approaches have not yet been validated on multi-site datasets where the lack of data standardization presents significant challenges. Moreover, due to the demand for model interpretability in healthcare,⁵² it is likely that more traditional approaches to modeling will remain relevant. Thus, we assert that it is still essential to develop better recommendations for reporting data standardization procedures used when modeling with multi-site datasets extracted from EHRs.

Limitations

This study had several limitations that should be addressed in future work. First, as the LOINC mapping utilized was part of an ongoing project at UPMC, only a partial LOINC mapping was available at the time of this study. Therefore, we chose to exclude from our analysis any laboratory tests that did not have a LOINC mapping. This allowed for a fair comparison of model performance with and without standardization to LOINC across the same set of laboratory tests. Alternatively, we could have utilized the local laboratory test codes when a LOINC mapping was unavailable, but we felt that this approach would introduce too much bias against LOINC standardization due to the partially complete mapping. This alternative approach would be appropriate to utilize once the UPMC team has finished the LOINC mapping process. Thus, our conclusions are based only on a subset of laboratory data; however, this subset captured a large portion of all laboratory test results in our dataset (∼64% of all test results). We therefore believe that an analysis based on a complete LOINC mapping would yield similar results, but plan to evaluate this idea in future work when a complete mapping is available.

Additionally, as the partial mapping was not originally generated for research purposes, intercoder agreement and false positive mappings were not tracked. Thus, a formal validation of the mapping process was unable to be performed. It would be beneficial to validate our claims using more rigorously tested mapping approaches; however, it would take significant time and expertise to complete such mappings. Moreover, the two coders on the mapping team were highly qualified, thoughtfully selected subject matter experts and the accuracy of these individuals working together to map codes was expected to be high. The mapping team subjectively estimated that less than 5% of the initial codes resulted in discrepancies that needed to be reviewed, and they were confident in the accuracy of their approach (ie, it was unlikely that false positive mappings would have occurred).

Finally, our definition of readmission included both planned and unplanned visits and we only examined a single prediction task for a specific patient population. We note that our claims may not be valid for other patient populations or for other prediction tasks. Future studies examining the impact of standardizing to LOINC on prediction performance should include in a variety of population and prediction tasks and utilize all available laboratory test results. The impact of standardizing other EHR data types on predictive model performance should be also explored. Such studies could provide further support for the need for detailed reporting on standardization procedures in predictive modeling studies.

CONCLUSION

This study investigated the impact of standardizing local laboratory codes to LOINC on predictive model performance in a multi-site dataset. We quantitatively demonstrated that standardizing to LOINC significantly improves predictive performance across a variety of feature selection and modeling techniques. Based on our findings, we have argued for the need for detailed reporting of data standardization procedures in predictive modeling, especially in studies leveraging multi-site datasets extracted from EHRs.

SUPPLEMENTARY MATERIAL

Supplementary material is available at Journal of the American Medical Informatics Association online.

CONTRIBUTORS

AB and FT designed this study. AB executed the study design and prepared the manuscript. VR provided substantial assistance in executing the study design and provided revisions to the manuscript. TG provided the LOINC mappings and critical insights into the mapping process. FT oversaw the research and provided revisions to the manuscript. All authors reviewed and approved the final manuscript version.