SPELL: A scalable NLP method using regular expressions and large language models for clinical information extraction.

BACKGROUND AND

objectivesElectronic health records (EHRs) contain valuable information for research and decision-making, but much resides in unstructured notes that are challenging to analyze at scale. We developed SPELL (Snippet-Primed rEgex LLM Pipeline), a scalable natural language processing workflow that combines regular-expression-based snippet retrieval with locally hosted large language model (LLM) inference to extract structured variables from large collections of clinical narratives.

methodsSPELL uses task-specific regular expressions to retrieve short context windows ("snippets") from unstructured texts and applies task-prompted LLM inference on snippets rather than full documents. All processing occurs within institutional computing environments. Accuracy was evaluated on randomly sampled, clinician-annotated benchmark sets of 50 documents per obstetric task, with separate retrieval-recall audits of 20 regex-negative documents per task. We evaluated accuracy and efficiency across three obstetric information-extraction tasks: numerical value (blood loss volume), date (estimated due date), and diagnosis (hemolysis, elevated liver enzymes, and low platelets [HELLP] syndrome). We quantified computational scalability using elapsed time, out-of-memory events, energy consumed, and GPU telemetry, and audited retrieval recall using clinician-annotated regex-negative notes enriched with relevant structured metadata. Generalizability was assessed on the public MT Samples corpus (5013 notes across 40 specialties) for ventricular tachycardia detection.

resultsSPELL processed 31 million clinical notes spanning 1976-2024 from eight hospitals. Snippet-based inference reduced processing time by 71-87% versus full-document LLM inference and by >95% versus manual physician annotation. On the 50-document benchmark sets, snippet-based evaluation achieved 98% exact-match accuracy for blood-loss extraction, 92% exact-match accuracy for estimated-due-date extraction, and 94% accuracy with an F1-score of 0.97 for HELLP syndrome classification. As an exploratory external evaluation on MT Samples, ventricular tachycardia detection achieved 84% accuracy and an F1-score of 0.67.

conclusionsA hybrid regex-snippet-LLM pipeline can enable accurate and computationally efficient extraction from unstructured EHR narratives.