Metabolomics Data Structure Handling

metabolomics-data-structure-handling

Summary

Load, organize, and access metabolomics assays within a SummarizedExperiment object to enable coordinated analysis across raw, imputed, and normalized data matrices. This skill ensures correct indexing of multiple assay layers (e.g., rawImpute, loessShort_concatenate) and their associated batch metadata for downstream visualization and batch-effect assessment.

When to use

You have metabolomics data already formatted as a SummarizedExperiment object containing multiple assays (raw counts, log-transformed, imputed, or normalized versions) and need to access specific assay layers alongside batch annotation metadata to perform comparisons (e.g., before/after normalization PCA plots, batch-effect diagnostics). Use this skill when transitioning between preprocessing stages that produce new assays stored in the same object.

When NOT to use

• Data is stored as separate CSV/TSV files or feature tables without integrated sample metadata—use file I/O and data frame merging instead.
• You only need a single assay snapshot and do not need to compare multiple normalization or processing states.
• The SummarizedExperiment object has not been formally constructed (e.g., rows and columns are not properly labeled, or colData lacks batch identifiers).

Inputs

• SummarizedExperiment object containing multiple metabolomics assays
• assay names as strings (e.g., 'rawImpute', 'loessShort_concatenate')
• batch metadata column name (e.g., 'batch_info')

Outputs

• Extracted assay matrix (rows = metabolites, columns = samples)
• Associated batch_info vector or colData subset
• Verified SummarizedExperiment object ready for downstream analysis

How to apply

Load the SummarizedExperiment object into R using the SummarizedExperiment library. Access individual assays by name using assay(object, 'assayName') or assays(object) to inspect all available layers. Verify the structure: rows = metabolites, columns = samples, with consistent metadata columns (e.g., batch_info) in colData(). Extract the appropriate assay (e.g., rawImpute for pre-normalization, loessShort_concatenate for post-normalization) along with its corresponding batch_info column from colData(). Use these paired objects as inputs to downstream functions like hRUV::plotPCA, ensuring the assay matrix dimensions and sample order align with metadata annotations.

Related tools

• SummarizedExperiment (Container class for organizing metabolomics assays (raw, imputed, normalized) alongside sample metadata and batch annotations; enables coordinated subsetting and cross-assay comparisons) — https://bioconductor.org/packages/SummarizedExperiment
• hRUV (Consumes SummarizedExperiment objects and accesses specific named assays (e.g., rawImpute, loessShort_concatenate) for normalization workflows and batch-effect visualization via plotPCA) — https://github.com/SydneyBioX/hRUV
• dplyr (Data manipulation utility for filtering, selecting, and reshaping assay matrices and metadata after extraction from SummarizedExperiment) — https://dplyr.tidyverse.org

Examples

library(SummarizedExperiment); dat <- readRDS('hruv_summarized_experiment.rds'); raw_assay <- assay(dat, 'rawImpute'); batch_info <- colData(dat)$batch_info; normalized_assay <- assay(dat, 'loessShort_concatenate')

Evaluation signals

• assay(object, 'assayName') returns a numeric matrix with metabolite rows and sample columns (no NA row/column names).
• colData(object) contains batch_info column with correct batch labels matching the number of samples in all assays.
• Assay dimensions are consistent across all named assays (same number of columns; row counts may differ if metabolites were filtered).
• Subsetting by batch_info (e.g., colData(object)$batch_info == 'batch_1') correctly indexes corresponding columns in the assay matrix.
• Downstream functions (e.g., hRUV::plotPCA) execute without dimension-mismatch errors and produce outputs colored/faceted by batch metadata as intended.

Limitations

• SummarizedExperiment assumes rectangular structure; metabolites missing in specific batches must be handled via imputation or explicit filtering before object construction.
• Assay names and metadata column names are case-sensitive; typos in assay('object', 'wrongName') will return NULL without warning.
• The object does not enforce data type consistency across assays (e.g., log-transformed vs. raw counts); users must track transformations manually or document them in metadata.
• Large metabolomics datasets (10,000+ metabolites, 1,000+ samples) may cause memory overhead if all assays are loaded simultaneously; consider subsetting by metabolite or batch before analysis.

Evidence

• [readme] The data is already formatted in to a SummarizedExperiment object: "The data is already formatted in to a SummarizedExperiment object"
• [full_text] Load the cleaned and imputed metabolomics data (rawImpute assay) from the SummarizedExperiment object: "Load the cleaned and imputed metabolomics data (rawImpute assay) from the SummarizedExperiment object"
• [full_text] Load the normalized metabolomics data (loessShort_concatenate assay) from the same SummarizedExperiment object: "Load the normalized metabolomics data (loessShort_concatenate assay) from the same SummarizedExperiment object"
• [full_text] Generate a PCA plot using hRUV::plotPCA with rawImpute assay colored by batch_info to visualize the pre-normalization batch effect: "Generate a PCA plot using hRUV::plotPCA with rawImpute assay colored by batch_info to visualize the pre-normalization batch effect"
• [readme] library(SummarizedExperiment)... The data is deposited at https://github.com/SydneyBioX/BioHEART_metabolomics: "library(SummarizedExperiment); The data is deposited at https://github.com/SydneyBioX/BioHEART_metabolomics"