Skip to main content

Cytogenetics

Assess Band Resolution

Band resolution refers to the level of detail visible on the chromosomes. It is determined by the degree of chromosomal condensation. In prophase (early mitosis), chromosomes are long and show many fine bands. As mitosis progresses to metaphase, chromosomes contract/shorten, and fine bands fuse into larger blocks. The “Banding Level” is a formal metric (e.g., 300, 400, 550, 850) defined by ISCN, based on the total number of visible bands in a haploid set

Why It Matters

  • Diagnostic Sensitivity
    • 300-400 Bands (Routine): Adequate for counting aneuploidy (Trisomy 21) or spotting large translocations (CML)
    • 550-850 Bands (High Resolution): Required to see Microdeletions (e.g., Prader-Willi 15q11.2, Velocardiofacial 22q11.2). Performing a microdeletion analysis on 400-band chromosomes is a technical error (False Negative risk)

How to Assess Resolution (The “Sentinel” Chromosomes)

Laboratory scientists estimate resolution by looking at specific chromosomes that show distinct splitting of bands as they elongate

Chromosome 10

  • Low Res (300): The q-arm has 3 dark bands
  • Medium Res (400): The top two bands fuse? No, usually distinct
  • High Res (550): The three dark bands on the q-arm split
    • Proximal band splits into 2
    • Middle band splits into 2
    • Distal band splits

Chromosome 16 (The Standard Check)

  • Low Res (300): The q-arm looks like a single solid dark block
  • Medium Res (400): The q-arm splits into a distinct dark band (16q22) and a lighter distal region
  • High Res (550): The dark band at 16q22 is clearly separated from the heterochromatin (16q12)

Chromosome 18

  • Low Res (300): Solid dark
  • Medium Res (400): Distinct light bands appear on p-arm and q-arm
  • High Res (550): Detailed sub-bands visible within the dark regions

ISCN Banding Levels

  • < 300 Bands: Poor. Condensed, “stubby” chromosomes. Bands are fused. Only suitable for basic counting (aneuploidy)
  • 400 Bands: Standard/Routine. Suitable for most oncology (bone marrow) and prenatal aneuploidy cases. The bands on 10q are distinct but solid
  • 550 Bands: High Resolution (Routine Blood). Required for constitutional structural analysis. The bands on 10q and 11q start to split into sub-bands
  • 850 Bands: Prometaphase. extremely long chromosomes. Used for specialized high-res studies (rarely done now, replaced by Microarray)

Reporting Resolution

The laboratory report usually states the resolution achieved

  • Phrasing: “Analysis performed at the 400-band level.”
  • Implication: This protects the lab. If a microdeletion is missed, the lab can argue, “This deletion is typically 2Mb, which is invisible at the 400-band level reported.”

Factors Affecting Resolution

  • Colcemid Time: Colcemid arrests cells in metaphase and condenses chromosomes
    • Too Long: Super-short, black chromosomes (<300 bands)
    • Too Short: Long, tangled chromosomes (850 bands, but hard to analyze)
  • Cell Type
    • Blood (Lymphocytes): Easily synchronized to yield 550+ bands
    • Bone Marrow: Harder to synchronize. Usually yields 300–400 bands
    • Amniocytes: Variable, usually 400–500 bands
  • Synchronization (MTX/Thymidine)
    • Chemical blocking (Methotrexate) followed by release (Thymidine) creates a wave of early mitotic cells (prometaphase), yielding high resolution

Troubleshooting Low Resolution

  • If the case requires high resolution (e.g., “Rule out Prader-Willi”) but the slides only have 400-band cells:
    • Do not report Negative.
    • Action: State “Resolution limited to 400 bands. This analysis cannot exclude microdeletions. Recommend FISH or Microarray.”