Use this as a morphology communication lexicon for acute chest CT, never as a binary SARS-CoV-2 test or severity score. Preserve timing, laboratory context, mixed processes, complications and the 2025 post-COVID terminology boundary.
{
"categories": [
{
"outcome_code": "typical",
"label": "typical_appearance",
"identifier": "Cov19Typ",
"meaning": "pattern_frequently_and_more_specifically_reported_with_acute_COVID_19_pneumonia_but_not_diagnostic"
},
{
"outcome_code": "indeterminate",
"label": "indeterminate_appearance",
"identifier": "Cov19Ind",
"meaning": "pattern_reported_with_COVID_19_but_without_the_more_specific_distribution_or_morphology"
},
{
"outcome_code": "atypical",
"label": "atypical_appearance",
"identifier": "Cov19Aty",
"meaning": "pattern_uncommon_for_COVID_19_and_more_suggestive_of_an_alternative_or_additional_process"
},
{
"outcome_code": "negative",
"label": "negative_for_pneumonia",
"identifier": "Cov19Neg",
"meaning": "no_parenchymal_abnormality_attributable_to_pneumonia_on_this_CT"
}
],
"applicability": {
"use_for": "Standardized communication of chest CT morphology potentially attributable to acute COVID-19 pneumonia when the examination has another valid clinical indication or the findings are encountered incidentally.",
"classification_unit": "one_diagnostic_chest_CT_examination_at_one_timepoint",
"required_inputs": [
"CT_date",
"symptom_onset_and_current_clinical_context_when_available",
"SARS_CoV_2_test_status_when_available",
"thin_section_lung_images",
"ground_glass_and_consolidation_morphology",
"distribution",
"organizing_pneumonia_features",
"atypical_features",
"complications_and_alternative_diagnoses"
],
"outside_scope": [
"screening_asymptomatic_people_by_CT",
"first_line_confirmation_or_exclusion_of_SARS_CoV_2_infection",
"pneumonia_extent_or_severity_score",
"treatment_or_isolation_decision_from_category_alone",
"post_COVID_residual_abnormality_classification",
"chest_radiograph_category"
]
},
"acquisition_and_adequacy_gate": {
"preferred_images": "Use diagnostic-quality thin-section axial lung reconstructions with multiplanar review and full lung coverage; contrast administration is determined by the actual CT indication rather than by this lexicon.",
"adequacy_checks": [
"complete_lung_coverage",
"motion_does_not_obscure_ground_glass",
"appropriate_lung_windows",
"comparison_with_prior_imaging",
"knowledge_of_supine_or_prone_and_inspiratory_limitations"
],
"nonclassifiable_rule": "If motion, dependent atelectasis, incomplete coverage or another artifact prevents confident pneumonia-pattern assessment, return technically limited rather than converting uncertainty into indeterminate appearance."
},
"feature_model": {
"opacity": [
"ground_glass_opacity",
"consolidation",
"ground_glass_with_superimposed_interlobular_and_intralobular_lines_or_crazy_paving"
],
"distribution": [
"peripheral",
"posterior",
"bilateral",
"multifocal",
"rounded",
"lower_lung_predominant",
"diffuse_or_no_specific_distribution",
"perihilar",
"unilateral"
],
"organizing_pneumonia_patterns": [
"reverse_halo_or_atoll_sign",
"perilobular_opacity",
"other_organizing_pneumonia_pattern"
],
"atypical_features": [
"isolated_lobar_or_segmental_consolidation_without_ground_glass",
"discrete_centrilobular_nodules_or_tree_in_bud",
"lung_cavitation",
"smooth_interlobular_septal_thickening_with_pleural_effusion"
],
"separately_report": [
"extent_and_lobar_distribution",
"emphysema_or_preexisting_diffuse_lung_disease",
"airway_disease",
"pleural_effusion",
"lymphadenopathy",
"pulmonary_embolism_when_evaluated",
"secondary_infection_or_aspiration",
"other_urgent_findings"
]
},
"assignment_algorithm": [
{
"if": "no_ground_glass_no_consolidation_and_no_other_parenchymal_abnormality_attributable_to_pneumonia",
"output_code": "negative"
},
{
"if": "peripheral_bilateral_ground_glass_with_or_without_consolidation_or_crazy_paving",
"output_code": "typical"
},
{
"if": "multifocal_rounded_ground_glass_with_or_without_consolidation_or_crazy_paving",
"output_code": "typical"
},
{
"if": "reverse_halo_or_other_organizing_pneumonia_pattern_in_the_appropriate_acute_context",
"output_code": "typical"
},
{
"if": "typical_features_absent_AND_multifocal_diffuse_perihilar_or_unilateral_ground_glass_with_or_without_consolidation_lacking_specific_distribution",
"output_code": "indeterminate"
},
{
"if": "typical_features_absent_AND_few_small_nonrounded_nonperipheral_ground_glass_opacities",
"output_code": "indeterminate"
},
{
"if": "typical_and_indeterminate_features_absent_AND_one_or_more_defined_atypical_features_predominate",
"output_code": "atypical"
}
],
"category_and_mixed_pattern_rules": {
"typical_is_not_positive_test": "Typical describes CT morphology and cannot confirm SARS-CoV-2 infection. Influenza, other viral pneumonias, acute lung injury and organizing pneumonia from drug toxicity, connective-tissue disease or idiopathic causes can be indistinguishable.",
"indeterminate_is_not_equivocal_test_result": "Indeterminate means the CT morphology lacks the more characteristic pattern; it is not a laboratory-test state or a fixed numeric probability.",
"atypical_is_not_COVID_exclusion": "Atypical favors an alternative or additional process but a patient can still have SARS-CoV-2 infection, including a simultaneous bacterial, aspiration or other infection.",
"negative_is_not_infection_exclusion": "Negative means no CT pneumonia pattern at this time. CT can be normal early in infection and cannot exclude SARS-CoV-2.",
"mixed_pattern_rule": "When typical and atypical findings coexist, describe both processes and assess whether superimposed infection, aspiration or another cause is likely. Do not discard the atypical finding merely to force a single clean category.",
"category_probability_rule": "Do not attach a universal likelihood to any category. Predictive value changes with community prevalence, exposure, symptoms, disease timing, vaccination, circulating variant and competing diagnoses."
},
"communication_and_clinical_context": {
"requested_assessment": "Use the standardized category and an evidence-based paraphrase of the suggested reporting language while retaining the observed findings and relevant differential diagnoses.",
"incidental_pattern": "If acute infection is unexpectedly suspected, communicate according to current local critical-result and infection-control policy. The original pandemic-era document recommended direct provider discussion and facility notification; contemporary action must follow current institutional rules.",
"laboratory_boundary": "Specific diagnosis requires current clinical and virologic evaluation. The category cannot replace an indicated SARS-CoV-2 assay or overrule a confirmed alternative diagnosis.",
"imaging_indication_boundary": "CT should not be ordered solely for routine COVID-19 screening or first-line diagnosis. Use it for an independent clinical indication and evaluate the full chest examination."
},
"temporal_and_current_use_boundaries": {
"acute_timing": "The four categories were created for acute pneumonia. A negative examination is especially limited near symptom onset, while organizing-pneumonia morphology can emerge later in the acute course.",
"vaccinated_and_variant_context": "Later research showed that the true-positive rate of the typical category varied with vaccination status, epidemic peaks and variants. Preserve date and clinical context rather than applying a 2020 probability as timeless.",
"post_COVID_boundary": "For persistent or progressive respiratory symptoms and residual abnormalities months after infection, use current post-COVID terminology and indication guidance; do not label residual bands, reticulation or air trapping with the acute four-category scheme.",
"severity_boundary": "The category ranks typicality, not burden or physiologic severity. Record extent, oxygenation, respiratory status and complications separately."
},
"differential_diagnosis_guard": {
"typical_pattern": [
"influenza_or_other_viral_pneumonia",
"organizing_pneumonia",
"drug_toxicity",
"connective_tissue_disease_related_lung_injury",
"other_acute_lung_injury"
],
"indeterminate_pattern": [
"hypersensitivity_pneumonitis",
"Pneumocystis_pneumonia",
"diffuse_alveolar_hemorrhage",
"pulmonary_edema",
"other_infection_or_inflammation"
],
"atypical_pattern": [
"bacterial_lobar_pneumonia",
"aspiration",
"tuberculosis_or_nontuberculous_mycobacteria",
"septic_emboli",
"necrotizing_infection",
"cardiogenic_edema"
],
"full_exam_rule": "Do not stop after assigning the lexicon. Report the leading alternative, coexisting process, complication and any urgent noninfectious finding supported by the CT."
},
"management_and_risk_context": {
"management_rule": "This reporting category does not prescribe testing, isolation, admission, antiviral therapy, antibiotics or follow-up CT. Those actions depend on current public-health guidance, symptoms, oxygenation, laboratory results, comorbidity and the actual imaging indication.",
"diagnostic_risk": "Overcalling typical morphology as confirmed COVID-19 can obscure treatable mimics; using atypical or negative morphology to exclude infection can miss true acute disease.",
"radiation_and_workflow_risk": "Unindicated CT adds radiation, transport and resource use. It should not be generated simply because a structured category exists."
},
"output_contract": [
"RSNA_2020_acute_CT_reporting_category_and_identifier",
"CT_date_technique_quality_and_comparison",
"observed_opacity_morphology_distribution_and_extent",
"typical_indeterminate_atypical_or_negative_with_trigger",
"mixed_pattern_and_alternative_or_superimposed_processes",
"SARS_CoV_2_test_and_symptom_timing_when_provided",
"separate_severity_complication_and_urgent_findings",
"no_fixed_probability_no_CT_only_diagnosis_and_no_autonomous_management"
],
"missing_input_behavior": [
"If clinical timing, exposure, vaccination or test status is absent, assign only the supported morphology category and state that predictive interpretation is unavailable.",
"If image quality prevents assessment, return technically limited rather than negative or indeterminate.",
"If both typical and atypical patterns are present, preserve both and the possibility of dual pathology instead of selecting by a hidden priority rule.",
"If the examination is post-acute, with residual findings months after infection, withhold the acute category and route to post-COVID residual-abnormality terminology."
],
"interpretation_limits": [
"The consensus was based on literature available in March 2020 and was designed to standardize communication during the pandemic, not to establish a universal diagnostic test.",
"Category performance varies across prevalence, variants, vaccination, symptom timing and referral populations.",
"The four labels do not quantify pneumonia extent, predict outcome or replace clinical and laboratory diagnosis."
],
"supporting_sources": [
{
"role": "primary_consensus",
"citation": "Simpson et al. Radiol Cardiothorac Imaging. 2020;2:e200152",
"doi": "10.1148/ryct.2020200152",
"pmcid": "PMC7255403"
},
{
"role": "implementation_and_reader_performance",
"citation": "de Jaegere et al. Radiol Cardiothorac Imaging. 2020;2:e200276",
"doi": "10.1148/ryct.2020200276"
},
{
"role": "vaccinated_population_context",
"citation": "Polyakov et al. Radiology. 2023;306:e220680",
"doi": "10.1148/radiol.220680"
},
{
"role": "post_COVID_scope_boundary",
"citation": "Yoon et al. Radiology. 2025",
"doi": "10.1148/radiol.243374"
}
],
"source_locator": "Simpson et al. 2020, DOI 10.1148/ryct.2020200152, Table 1 and Categories for the four definitions and suggested language; Chest CT in COVID-19 Infection and Pros, Cons, and Limitations for timing, diagnostic and mixed-pattern safeguards; ACR 2020 position statement for the no-screening and no-first-line-diagnosis boundary; Polyakov et al. 2023 for vaccination, variant and prevalence dependence; Yoon et al. 2025 for the post-COVID residual-abnormality boundary."
}