---
title: "Biomarker Classification and Interpretation Guide"
description: "Biomarkers are measurable indicators of biological state or condition. In clinical decision support, biomarkers guide diagnosis, prognosis, treatment selection, and monitoring. This guide covers genomic, proteomic, and molecular biomarkers with emphasis on clinical actionability."
type: skill
canonical_url: https://claudary.paisolsolutions.com/skills/biomarker-classification
source: "Claudary"
difficulty: intermediate
author: "Claude Code Knowledge Pack"
date: 2026-07-10T11:08:27.776Z
license: CC-BY-4.0
attribution: "Biomarker Classification and Interpretation Guide — Claudary (https://claudary.paisolsolutions.com/skills/biomarker-classification)"
---

# Biomarker Classification and Interpretation Guide
Biomarkers are measurable indicators of biological state or condition. In clinical decision support, biomarkers guide diagnosis, prognosis, treatment selection, and monitoring. This guide covers genomic, proteomic, and molecular biomarkers with emphasis on clinical actionability.

## Overview

# Biomarker Classification and Interpretation Guide

## Overview

Biomarkers are measurable indicators of biological state or condition. In clinical decision support, biomarkers guide diagnosis, prognosis, treatment selection, and monitoring. This guide covers genomic, proteomic, and molecular biomarkers with emphasis on clinical actionability.

## Biomarker Categories

### Prognostic Biomarkers

**Definition**: Predict clinical outcome (survival, recurrence) regardless of treatment received

**Examples by Disease**

**Cancer**
- **Ki-67 index**: High proliferation (>20%) predicts worse outcome in breast cancer
- **TP53 mutation**: Poor prognosis across many cancer types
- **Tumor stage/grade**: TNM staging, histologic grade
- **LDH elevation**: Poor prognosis in melanoma, lymphoma
- **AFP elevation**: Poor prognosis in hepatocellular carcinoma

**Cardiovascular**
- **NT-proBNP/BNP**: Elevated levels predict mortality in heart failure
- **Troponin**: Predicts adverse events in ACS
- **CRP**: Inflammation marker, predicts cardiovascular events

**Infectious Disease**
- **HIV viral load**: Predicts disease progression if untreated
- **HCV genotype**: Predicts treatment duration needed

**Application**: Risk stratification, treatment intensity selection, clinical trial enrollment

### Predictive Biomarkers

**Definition**: Identify patients likely to benefit (or not benefit) from specific therapy

**Positive Predictive Biomarkers (Treatment Benefit)**

**Oncology - Targeted Therapy**
- **EGFR exon 19 del/L858R → EGFR TKIs**: Response rate 60-70%, PFS 10-14 months
- **ALK rearrangement → ALK inhibitors**: ORR 70-90%, PFS 25-34 months  
- **HER2 amplification → Trastuzumab**: Benefit only in HER2+ (IHC 3+ or FISH+)
- **BRAF V600E → BRAF inhibitors**: ORR 50%, PFS 6-7 months (melanoma)
- **PD-L1 ≥50% → Pembrolizumab**: ORR 45%, PFS 10 months vs 6 months (chemo)

**Oncology - Immunotherapy**
- **MSI-H/dMMR → Anti-PD-1**: ORR 40-60% across tumor types
- **TMB-high → Immunotherapy**: Investigational, some benefit signals
- **PD-L1 expression → Anti-PD-1/PD-L1**: Higher expression correlates with better response

**Hematology**
- **BCR-ABL → Imatinib (CML)**: Complete cytogenetic response 80%
- **CD20+ → Rituximab (lymphoma)**: Benefit only if CD20-expressing cells
- **CD33+ → Gemtuzumab ozogamicin (AML)**: Benefit in CD33+ subset

**Negative Predictive Biomarkers (Resistance/No Benefit)**
- **KRAS mutation → Anti-EGFR mAbs (CRC)**: No benefit, contraindicated
- **EGFR T790M → 1st/2nd-gen TKIs**: Resistance mechanism, use osimertinib
- **RAS/RAF wild-type required → BRAF inhibitors (melanoma)**: Paradoxical MAPK activation

### Diagnostic Biomarkers

**Definition**: Detect or confirm presence of disease

**Infectious Disease**
- **PCR for pathogen DNA/RNA**: SARS-CoV-2, HIV, HCV viral load
- **Antibody titers**: IgM (acute), IgG (prior exposure/immunity)
- **Antigen tests**: Rapid detection (strep, flu, COVID)

**Autoimmune**
- **ANA**: Screen for lupus, connective tissue disease
- **Anti-CCP**: Specific for rheumatoid arthritis
- **Anti-dsDNA**: Lupus, correlates with disease activity
- **ANCA**: Vasculitis (c-ANCA for GPA, p-ANCA for MPA)

**Cancer**
- **PSA**: Prostate cancer screening/monitoring
- **CA 19-9**: Pancreatic cancer, biliary obstruction
- **CEA**: Colorectal cancer monitoring
- **AFP**: Hepatocellular carcinoma, germ cell tumors

### Pharmacodynamic Biomarkers

**Definition**: Assess treatment response or mechanism of action

**Examples**
- **HbA1c**: Glycemic control in diabetes (target <7% typically)
- **LDL cholesterol**: Statin efficacy (target <70 mg/dL in high-risk)
- **Blood pressure**: Antihypertensive efficacy (target <130/80 mmHg)
- **Viral load suppression**: Antiretroviral efficacy (target <20 copies/mL)
- **INR**: Warfarin anticoagulation monitoring (target 2-3 for most indications)

## Genomic Biomarkers

### Mutation Analysis

**Driver Mutations (Oncogenic)**
- **Activating mutations**: Constitutive pathway activation (BRAF V600E, EGFR L858R)
- **Inactivating mutations**: Tumor suppressor loss (TP53, PTEN)
- **Hotspot mutations**: Recurrent positions (KRAS G12/G13, PIK3CA H1047R)
- **Variant allele frequency (VAF)**: Clonality (VAF ≈50% clonal, <10% subclonal)

**Resistance Mutations**
- **EGFR T790M**: Resistance to 1st/2nd-gen TKIs (40-60% of cases)
- **ALK G1202R, I1171N**: Resistance to early ALK inhibitors
- **ESR1 mutations**: Resistance to aromatase inhibitors (breast cancer)
- **RAS mutations**: Acquired resistance to anti-EGFR therapy (CRC)

**Mutation Detection Methods**
- **Tissue NGS**: Comprehensive genomic profiling, 300-500 genes
- **Liquid biopsy**: ctDNA analysis, non-invasive, serial monitoring
- **PCR-based assays**: Targeted hotspot detection, FDA-approved companion diagnostics
- **Allele-specific PCR**: High sensitivity for known mutations (cobas EGFR test)

### Copy Number Variations (CNV)

**Amplifications**
- **HER2 (ERBB2)**: Breast, gastric cancer → trastuzumab, pertuzumab
  - Testing: IHC (0, 1+, 2+, 3+) → FISH if 2+ (HER2/CEP17 ratio ≥2.0)
- **MET amplification**: NSCLC resistance mechanism → crizotinib, capmatinib
  - Cut-point: Gene copy number ≥5, GCN/CEP7 ratio ≥2.0
- **EGFR amplification**: Glioblastoma, some NSCLC
- **FGFR2 amplification**: Gastric cancer → investigational FGFR inhibitors

**Deletions**
- **PTEN loss**: Common in many cancers, predicts PI3K pathway activation
- **RB1 loss**: Small cell transformation, poor prognosis
- **CDKN2A/B deletion**: Cell cycle dysregulation
- **Homozygous deletion**: Complete loss of both alleles (more significant)

**Detection Methods**
- **FISH (Fluorescence In Situ Hybridization)**: HER2, ALK rearrangements
- **NGS copy number calling**: Depth of coverage analysis
- **SNP array**: Genome-wide CNV detection
- **ddPCR**: Quantitative copy number measurement

### Gene Fusions and Rearrangements

**Oncogenic Fusions**
- **ALK fusions** (NSCLC): EML4-ALK most common (60%), 20+ partners
  - Detection: IHC (D5F3 antibody), FISH (break-apart probe), NGS/RNA-seq
- **ROS1 fusions** (NSCLC, glioblastoma): CD74-ROS1, SLC34A2-ROS1, others
- **RET fusions** (NSCLC, thyroid): KIF5B-RET, CCDC6-RET
- **NTRK fusions** (many tumor types, rare): ETV6-NTRK3, others
  - Pan-cancer: Larotrectinib, entrectinib approved across tumor types
- **BCR-ABL** (CML, ALL): t(9;22), Philadelphia chromosome

**Fusion Partner Considerations**
- Partner influences drug sensitivity (EML4-ALK variant 3 more sensitive)
- 5' vs 3' fusion affects detection methods
- Intron breakpoints vary (RNA-seq more comprehensive than DNA panels)

**Detection Methods**
- **FISH break-apart probes**: ALK, ROS1, RET
- **IHC**: ALK protein overexpression (screening), ROS1
- **RT-PCR**: Targeted fusion detection
- **RNA-seq**: Comprehensive fusion detection, identifies novel partners

### Tumor Mutational Burden (TMB)

**Definition**: Number of somatic mutations per megabase of DNA

**Classification**
- **TMB-high**: ≥10 mutations/Mb (some definitions ≥20 mut/Mb)
- **TMB-intermediate**: 6-9 mutations/Mb
- **TMB-low**: <6 mutations/Mb

**Clinical Application**
- **Predictive for immunotherapy**: Higher TMB → more neoantigens → better immune response
- **FDA approval**: Pembrolizumab for TMB-H (≥10 mut/Mb) solid tumors (2020)
- **Limitations**: Not validated in all tumor types, assay variability

**Tumor Types with Typically High TMB**
- Melanoma (median 10-15 mut/Mb)
- NSCLC (especially smoking-associated, 8-12 mut/Mb)
- Urothelial carcinoma (8-10 mut/Mb)
- Microsatellite instable tumors (30-50 mut/Mb)

### Microsatellite Instability (MSI) and Mismatch Repair (MMR)

**Classification**
- **MSI-high (MSI-H)**: Instability at ≥2 of 5 loci or ≥30% of markers
- **MSI-low (MSI-L)**: Instability at <2 of 5 loci
- **Microsatellite stable (MSS)**: No instability

**Mismatch Repair Status**
- **dMMR (deficient)**: Loss of MLH1, MSH2, MSH6, or PMS2 by IHC
- **pMMR (proficient)**: Intact expression of all four MMR proteins

**Clinical Significance**
- **MSI-H/dMMR Tumors**: 3-5% of most solid tumors, 15% of colorectal cancer
- **Immunotherapy Sensitivity**: ORR 30-60% to anti-PD-1 therapy
  - Pembrolizumab FDA-approved for MSI-H/dMMR solid tumors (2017)
  - Nivolumab ± ipilimumab approved
- **Chemotherapy Resistance**: MSI-H CRC does not benefit from 5-FU adjuvant therapy
- **Lynch Syndrome**: Germline MMR mutation if MSI-H + young age + family history

**Testing Algorithm**
```
Colorectal Cancer (all newly diagnosed):
1. IHC for MMR proteins (MLH1, MSH2, MSH6, PMS2)
   ├─ All intact → pMMR (MSS) → Standard chemotherapy if indicated
   │
   └─ Loss of one or more → dMMR (likely MSI-H)
      └─ Reflex MLH1 promoter hypermethylation test
         ├─ Methylated → Sporadic MSI-H, immunotherapy option
         └─ Unmethylated → Germline testing for Lynch syndrome
```

## Expression Biomarkers

### Immunohistochemistry (IHC)

**PD-L1 Expression (Immune Checkpoint)**
- **Assays**: 22C3 (FDA), 28-8, SP263, SP142 (some differences in scoring)
- **Scoring**: Tumor Proportion Score (TPS) = % tumor cells with membrane staining
  - TPS <1%: Low/negative
  - TPS 1-49%: Intermediate
  - TPS ≥50%: High
- **Combined Positive Score (CPS)**: (PD-L1+ tumor + immune cells) / total tumor cells × 100
  - Used for some indications (e.g., CPS ≥10 for pembrolizumab in HNSCC)

**Hormone Receptors (Breast Cancer)**
- **ER/PR Positivity**: ≥1% nuclear staining by IHC (ASCO/CAP guidelines)
  - Allred Score 0-8 (proportion + intensity) - historical
  - H-score 0-300 (percentage at each intensity) - quantitative
- **Clinical Cut-Points**:
  - ER ≥1%: Endocrine therapy indicated
  - ER 1-10%: "Low positive," may have lower benefit
  - PR loss with ER+: Possible endocrine resistance

**HER2 Testing (Breast/Gastric Cancer)**
```
IHC Initial Test:
├─ 0 or 1+: HER2-negative (no further testing)
│
├─ 2+: Equivocal → Reflex FISH testing
│  ├─ FISH+ (HER2/CEP17 ratio ≥2.0 OR HER2 copies ≥6/cell) → HER2-positive
│  └─ FISH- → HER2-negative
│
└─ 3+: HER2-positive (no FISH needed)
   └─ Uniform intense complete membrane staining in >10% of tumor cells

HER2-positive: Trastuzumab-based therapy indicated
HER2-low (IHC 1+ or 2+/FISH-): Trastuzumab deruxtecan eligibility (2022)
```

### RNA Expression Analysis

**Gene Expression Signatures (Breast Cancer)**

**Oncotype DX (21-gene assay)**
- **Recurrence Score (RS)**: 0-100
  - RS <26: Low risk → Endocrine therapy alone (most patients)
  - RS 26-100: High risk → Chemotherapy + endocrine therapy
- **Population**: ER+/HER2-, node-negative or 1-3 positive nodes
- **Evidence**: TAILORx trial (N=10,273) validated RS <26 can omit chemo

**MammaPrint (70-gene assay)**
- **Result**: High risk vs Low risk (binary)
- **Population**: Early-stage breast cancer, ER+/HER2-
- **Evidence**: MINDACT trial validated low-risk can omit chemo

**Prosigna (PAM50)**
- **Result**: Risk of Recurrence (ROR) score + intrinsic subtype
- **Subtypes**: Luminal A, Luminal B, HER2-enriched, Basal-like
- **Application**: Post-menopausal, ER+, node-negative or 1-3 nodes

**RNA-Seq for Fusion Detection**
- **Advantage**: Detects novel fusion partners, quantifies expression
- **Application**: NTRK fusions (rare, many partners), RET fusions
- **Limitation**: Requires fresh/frozen tissue or good-quality FFPE RNA

## Molecular Subtypes

### Glioblastoma (GBM) Molecular Classification

**Verhaak 2010 Classification (4 subtypes)**

**Proneural Subtype**
- **Characteristics**: PDGFRA amplification, IDH1 mutations (secondary GBM), TP53 mutations
- **Age**: Younger patients typically
- **Prognosis**: Better prognosis (median OS 15-18 months)
- **Treatment**: May benefit from bevacizumab less than other subtypes

**Neural Subtype**
- **Characteristics**: Neuron markers (NEFL, GABRA1, SYT1, SLC12A5)
- **Controversy**: May represent normal brain contamination
- **Prognosis**: Intermediate
- **Treatment**: Standard temozolomide-based therapy

**Classical Subtype**
- **Characteristics**: EGFR amplification (97%), chromosome 7 gain, chromosome 10 loss
- **Association**: Lacks TP53, PDGFRA, NF1 mutations
- **Prognosis**: Intermediate
- **Treatment**: May benefit from EGFR inhibitors (investigational)

**Mesenchymal Subtype**
- **Characteristics**: NF1 mutations/deletions, high expression of mesenchymal markers (CHI3L1/YKL-40)
- **Immune Features**: Higher macrophage/microglia infiltration
- **Subgroup**: Mesenchymal-immune-active (high immune signature)
- **Prognosis**: Poor prognosis (median OS 12-13 months)
- **Treatment**: May respond better to anti-angiogenic therapy, immunotherapy investigational

**Clinical Application**
```
GBM Molecular Subtyping Report:

Patient Cohort: Mesenchymal-Immune-Active Subtype (n=15)

Molecular Features:
- NF1 alterations: 73% (11/15)
- High YKL-40 expression: 100% (15/15)
- Immune gene signature: Elevated (median z-score +2.3)
- CD163+ macrophages: High density (median 180/mm²)

Treatment Implications:
- Standard therapy: Temozolomide-based (Stupp protocol)
- Consider: Bevacizumab for recurrent disease (may have enhanced benefit)
- Clinical trial: Immune checkpoint inhibitors ± anti-angiogenic therapy
- Prognosis: Median OS 12-14 months (worse than proneural)

Recommendation:
Enroll in combination immunotherapy trial if eligible, otherwise standard therapy
with early consideration of bevacizumab at progression.
```

### Breast Cancer Intrinsic Subtypes

**PAM50-Based Classification**

**Luminal A**
- **Characteristics**: ER+, HER2-, low proliferation (Ki-67 <20%)
- **Gene signature**: High ER-related genes, low proliferation genes
- **Prognosis**: Best prognosis, low recurrence risk
- **Treatment**: Endocrine therapy alone usually sufficient
- **Chemotherapy**: Rarely needed unless high-risk features

**Luminal B**
- **Characteristics**: ER+, HER2- or HER2+, high proliferation (Ki-67 ≥20%)
- **Subtypes**: Luminal B (HER2-) and Luminal B (HER2+)
- **Prognosis**: Intermediate prognosis
- **Treatment**: Chemotherapy + endocrine therapy; add trastuzumab if HER2+

**HER2-Enriched**
- **Characteristics**: HER2+, ER-, PR-
- **Gene signature**: High HER2 and proliferation genes, low ER genes
- **Prognosis**: Poor if untreated, good with HER2-targeted therapy
- **Treatment**: Chemotherapy + trastuzumab + pertuzumab

**Basal-Like**
- **Characteristics**: ER-, PR-, HER2- (triple-negative), high proliferation
- **Gene signature**: Basal cytokeratins (CK5/6, CK17), EGFR
- **Overlap**: 80% concordance with TNBC, but not identical
- **Prognosis**: Aggressive, high early recurrence risk
- **Treatment**: Chemotherapy (platinum, anthracycline), PARP inhibitors if BRCA-mutated
- **Immunotherapy**: PD-L1+ may benefit from pembrolizumab + chemotherapy

### Colorectal Cancer Consensus Molecular Subtypes (CMS)

**CMS1 (14%): MSI Immune**
- **Features**: MSI-high, BRAF mutations, strong immune activation
- **Prognosis**: Poor survival after relapse despite immune infiltration
- **Treatment**: Immunotherapy highly effe

---

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