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Skillsomicverse-bulk-metabol-preprocessing

omicverse-bulk-metabol-preprocessing

Data & analytics

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Updated 10-16 02:38

About this skill

OmicVerse Bulk Metabolomics Preprocessing

Goal

Take a metabolomics peak table (NMR or LC-MS; MetaboAnalyst CSV, generic wide CSV, or m/z/RT-coded LC-MS table) into an AnnData and run the canonical clean-up chain — load → impute → sample-normalize → feature-transform — plus optional MS-specific drift / batch / sample-outlier correction. The output is an analysis-ready AnnData with obs['group'] populated, raw intensities preserved as a layer, and the matrix in the right scale for the downstream skill (univariate stats vs. multivariate model).

Stay focused on preprocessing. Univariate differential / anova is a thin add-on shown for completeness; multivariate (PLS-DA, OPLS-DA), pathway enrichment (MSEA), biomarker selection, DGCA, and multi-omics MOFA are separate downstream skills.

Quick Workflow

  1. Pick the loader by file format and pass group_col — every downstream test reads adata.obs['group'].
  2. Impute missing values; method depends on the missingness mechanism (MAR → knn; MNAR / left-censored LC-MS → qrilc).
  3. Normalize each sample row to remove dilution effects — PQN is canonical.
  4. Transform each feature column for variance stabilization (log), and Pareto-scale on top only when a multivariate model will consume the matrix.
  5. For LC-MS with QC pools and injection order: run drift_correct (LOESS-on-QC) → serrf (RF on QC) → ov.bulk.batch_correction (ComBat on residual batch shifts), in that order.
  6. Run sample_qc on the corrected real samples to flag outliers; inspect manually before removing.
  7. Validate that adata.obs['group'] exists, no NaNs remain in adata.X, and raw matrix is preserved in a layer.

Interface Summary

Loaders (each returns AnnData with obs=samples, var=metabolites, obs['group'] populated):

  • ov.metabol.read_metaboanalyst(path, *, group_col, sample_col=None, transpose=False) — MetaboAnalyst-format CSV.
  • ov.metabol.read_wide(path, *, sample_col, group_col, ...) — generic wide CSV with explicit axes.
  • ov.metabol.read_lcms(path, *, feature_id_sep='/', sample_col=None, group_col=None, label_row=None, transpose=True) — LC-MS peak table; parses m/z and RT from feature IDs into var['mz'] / var['rt'].

Preprocessing chain (each is AnnData-in / AnnData-out and stashes the previous matrix into adata.layers when stash_raw=True):

  • ov.metabol.impute(adata, *, method='qrilc'|'knn'|'half_min'|'zero', missing_threshold=0.5, n_neighbors=5, q=0.01, seed=0).
  • ov.metabol.normalize(adata, *, method='pqn'|'tic'|'median'|'mstus', reference='median'|'mean', missing_threshold=0.5).
  • ov.metabol.transform(adata, *, method='log'|'glog'|'autoscale'|'pareto', pseudocount=1.0, stash_raw=True).

QC and MS-specific correction:

  • ov.metabol.cv_filter(adata, *, qc_mask=None, cv_threshold=0.30, across='qc'|'all') — drop high-CV features.
  • ov.metabol.blank_filter(adata, *, blank_mask, ratio=3.0) — drop features not above blank intensity.
  • ov.metabol.drift_correct(adata, *, injection_order, qc_mask, frac=0.5) — LOESS regression on QC samples vs. injection order.
  • ov.metabol.serrf(adata, *, qc_col, qc_label='QC', batch_col=None, top_k=10, n_estimators=100, min_qc_samples=5, layer=None, seed=0) — RF residual correction (Fan 2019). Adds var['cv_qc_raw'] and var['cv_qc_serrf'].
  • ov.bulk.batch_correction(adata, *, batch_key, key_added='batch_correction') — ComBat; corrected matrix lands at adata.layers['batch_correction'].
  • ov.metabol.sample_qc(adata, *, n_components=2, alpha=0.95, center=True, scale=True, layer=None) → pd.DataFrame with T2, DModX, T2_crit, DModX_crit, is_outlier.
  • ov.metabol.sample_qc_plot(qc_df, *, ax=None, figsize=(5, 4), normal_color, outlier_color) — Hotelling T² vs. DModX scatter with critical-value lines.

Univariate stats (kept here for completeness — they consume the log-transformed matrix, not the Pareto one):

  • ov.metabol.differential(adata, *, group_col='group', group_a, group_b, method='welch_t'|'mannwhitney'|'paired_t', layer=None, log_transformed=True) → pd.DataFrame with log2fc, pvalue, padj, mean_a, mean_b.
  • ov.metabol.anova(adata, *, group_col='group', groups=None, method='welch_anova'|'anova'|'kruskal', layer=None) → pd.DataFrame.
  • ov.metabol.volcano(deg, *, padj_thresh=0.05, log2fc_thresh=1.0, label_top_n=10, use_pvalue=False, clip_log2fc=None, ax=None, figsize=(5.5, 4.5)).

Chained class API:

  • ov.metabol.pyMetabo(adata, random_state=0) — keeps self.raw frozen, exposes .cv_filter / .drift_correct / .blank_filter / .impute / .normalize / .transform / .differential / .plsda / .opls_da (all chainable, all return self), plus .deg_table, .plsda_result, .vip_table(), .significant_metabolites(padj_thresh, log2fc_thresh).

Boundary

Keep this skill focused on the deterministic preprocessing chain plus MS-specific correction. Inside scope:

  • Loading any of the three peak-table formats.
  • Imputation, sample-normalization, feature-transformation in any order, with reproducible seeding.
  • LC-MS drift / SERRF / ComBat / sample-QC stack.
  • Demonstration that the pyMetabo chain produces identical numerics to the functional API (notebook asserts equality to machine precision).

Outside scope — separate skill:

  • PLS-DA / OPLS-DA model fitting, VIP scoring, S-plot (multivariate skill).
  • MSEA, mummichog, KEGG / LIPID MAPS / LION / HMDB ID mapping (pathway skill).
  • Biomarker panel + nested CV (biomarker skill).
  • DGCA differential correlation (correlation skill).
  • ASCA / mixed model / MEBA multi-factor designs (multifactor skill).
  • MOFA multi-view integration (multi-omics skill).
  • LIPID MAPS lipid parsing + class aggregation (lipidomics skill).

If a future tutorial only swaps the imputation method, normalization choice, or adds a new transformation primitive, update this skill rather than creating a duplicate.

Branch Selection

Imputer (impute(method=...)) by missingness mechanism

  • qrilc — left-censored / below-detection (LC-MS default). Draws from a truncated quantile-regressed normal.
  • knn — missing-at-random (sample mishandling, integration glitches). Sensitive to n_neighbors.
  • half_min — fast baseline, fills with feature half-minimum. Use only when distributional fidelity is not critical.
  • zero — only when zeros are structurally meaningful (e.g., truly absent metabolite); never the right call for LC-MS NaNs.

Sample normalizer (normalize(method=...))

  • pqn — Probabilistic Quotient Normalization. Default for both NMR and LC-MS.
  • tic — Total Ion Count. Only safe when the matrix is conserved (all samples on the same instrument run, same injection volume).
  • median — sample-median ratio. Robust to extreme features; less aggressive than PQN.
  • mstus — MS Total Useful Signal (LC-MS only — drops low-coverage features before TIC).

Feature transform (transform(method=...)) — sequence matters

  • log then downstream univariate stats (differential / anova). Pass log_transformed=True so fold-changes are computed on un-logged means.
  • log → pareto for downstream multivariate (plsda, opls_da, asca). Pareto centres each feature and divides by sqrt(SD) — preserves dominant signal structure.
  • autoscale (z-score) only if you specifically want unit variance per feature.
  • glog for very-low-intensity LC-MS features where log over-amplifies noise.

Correction stack (LC-MS only) — order matters

  1. drift_correct — injection-order drift via LOESS on QC. Skip if QC samples aren't interspersed.
  2. serrf — within-batch QC variability via Random Forest. Needs ≥5 QC pools per batch.
  3. ov.bulk.batch_correction (ComBat) — between-batch additive shifts. Use only if residual batch effect remains after SERRF and you have ≥3 samples per batch.

pyMetabo vs. functional API

  • Use pyMetabo for reproducible single-seed pipelines and when downstream code wants .deg_table / .plsda_result / .significant_metabolites() accessors.
  • Use the functional API when you want to inspect intermediate AnnData between every step or chain in / out of non-metabol code.

Input Contract

  • AnnData with obs=samples, var=metabolites (loaders enforce this — pass transpose=True if your file is the other way).
  • adata.obs must carry the factor column you'll pass as group_col; the loader writes it as obs['group'].
  • For LC-MS drift / SERRF: adata.obs needs sample_type (with a 'QC' label) and batch; for drift_correct it also needs an injection-order column (numeric, monotonic).
  • Missing values must be NaN, not zero (loaders handle this for the supported formats; check if you built the AnnData manually).
  • adata.X is dense float — sparse matrices are not supported by the imputers / SERRF.

Minimal Execution Patterns

# Functional one-shot (NMR / MetaboAnalyst CSV)
import omicverse as ov

ov.plot_set()
adata = ov.metabol.read_metaboanalyst('human_cachexia.csv', group_col='Muscle loss')
adata = ov.metabol.impute(adata, method='qrilc', q=0.01, seed=0)
adata = ov.metabol.normalize(adata, method='pqn')
log_adata = ov.metabol.transform(adata, method='log')
pareto_adata = ov.metabol.transform(log_adata, method='pareto', stash_raw=False)

deg = ov.metabol.differential(
    log_adata, group_col='group', group_a='cachexic', group_b='control',
    method='welch_t', log_transformed=True,
)
fig, ax = ov.metabol.volcano(deg, padj_thresh=0.10, log2fc_thresh=0.3, label_top_n=8)

# Chained class API — same numerics, more compact
m = (
    ov.metabol.pyMetabo(adata.copy())
      .impute(method='qrilc', seed=0)
      .normalize(method='pqn')
      .transform(method='log')
      .differential(group_col='group', group_a='cachexic', group_b='control',
                    method='welch_t', log_transformed=True)
      .transform(method='pareto', stash_raw=False)
)
m.deg_table.head()
m.significant_metabolites(padj_thresh=0.10, log2fc_thresh=0.3)

# LC-MS correction stack
adata = ov.metabol.drift_correct(
    adata,
    injection_order='order',
    qc_mask=(adata.obs['sample_type'] == 'QC').to_numpy(),
    frac=0.5,
)
adata = ov.metabol.serrf(
    adata,
    qc_col='sample_type', qc_label='QC',
    batch_col='batch',
    top_k=8, n_estimators=100, seed=0,
)
ov.bulk.batch_correction(adata, batch_key='batch', key_added='batch_correction')
# Corrected matrix at adata.layers['batch_correction']

real = adata[adata.obs['sample_type'] == 'real'].copy()
qc_df = ov.metabol.sample_qc(real, n_components=3, alpha=0.95)
ov.metabol.sample_qc_plot(qc_df)
flagged = qc_df.index[qc_df['is_outlier']].tolist()

Validation

  • 'group' in adata.obs.columns after loading — if missing, the loader's group_col was wrong.
  • not np.isnan(adata.X).any() after impute(...) — if NaNs remain, missingness exceeded missing_threshold for at least one column; either lower the threshold or drop those features.
  • After normalize(method='pqn'), row-median dispersion (max/min) should drop versus the raw matrix — if it doesn't, picked the wrong method.
  • After transform(method='pareto', ...), every feature's mean should be ≈0 — if not, Pareto wasn't applied (likely because data wasn't log-transformed first).
  • After serrf, adata.var['cv_qc_serrf'] should be lower than cv_qc_raw for most features; the scatter cv_qc_raw vs cv_qc_serrf should sit below the diagonal.
  • After sample_qc, manually inspect samples flagged with is_outlier=True before dropping — biological outliers (rare phenotypes) and technical outliers look identical here.
  • The pyMetabo chain and the functional API must agree to machine precision when the same seed and method choices are used (assert (m.deg_table['pvalue'].values == deg['pvalue'].values).all()).
  • If you only validated the chain on the cachexia NMR dataset, do not claim LC-MS correctness; both ingest paths share imputation / normalization but diverge on read_lcms parsing and on the drift / SERRF / ComBat stack.

Resource Map

  • See reference.md for copy-paste-ready snippets for each stage.
  • See references/source-grounding.md for the live API signatures and the docstring-supplementation log (pyMetabo chainable methods + plotting helpers were docstring-empty; this skill drove their backfill).
  • For multivariate (PLS-DA / OPLS-DA / VIP / S-plot) downstream of this preprocessing, see the multivariate skill (separate).
  • For pathway enrichment (MSEA ORA / GSEA, mummichog), see the pathway-multifactor skill (separate).
  • For biomarker selection with nested CV, see the biomarker skill.
  • For multi-omics integration (MOFA on metabolomics + RNA-seq), see the multi-omics MOFA skill.

Examples

  • "Load human_cachexia.csv (MetaboAnalyst format, factor column Muscle loss), impute with QRILC, PQN-normalize, log-transform, run a Welch t-test for cachexic vs. control, and plot a volcano with padj<0.10."
  • "My LC-MS run has injection-order drift and 3 batches with QC pools — apply drift_correct + SERRF + ComBat in that order; confirm SERRF dropped median QC CV% by at least 30 %."
  • "Run sample QC on the real-sample subset and list any samples above the 0.95 critical Hotelling line."

References