Automated Extraction and Specimen Considerations

Same chemistry, different hands

Instruments run the same lyse, bind, wash, elute sequence a person does at the bench - with fixed volumes and timing and a mechanical way to separate the bound nucleic acid from the liquid ¹.

Magnetic silica beads

Most automated extraction replaces the column with paramagnetic beads: tiny particles with a silica surface and a magnetic core. In chaotropic salt, nucleic acid binds the beads; a magnet pulls the beads to the side of the well while liquid is removed; release the magnet and the beads resuspend in the next wash, then in elution buffer ¹.

Two instrument designs: a magnetic rod (in a disposable sleeve) carries beads from well to well, or the plate sits on fixed magnets while a pipetting head exchanges liquids.

Why automate:

• Throughput: a full plate at once.
• Reproducibility: the same volumes and timing in every well, every run - which quantitative assays depend on ².
• Fewer open transfers: less chance of carryover between samples.

Cartridges and sample-to-answer

Cartridge systems seal all extraction reagents in a single-use cartridge; the operator adds specimen and loads it. Sample-to-answer systems also amplify and detect inside the same closed cartridge, so the tube is never opened between specimen and result ¹. Respiratory and GI multiplex panels and rapid MRSA or C. difficile tests work this way, which suits near-patient testing and small labs.

Specimen by specimen

FFPE in practice: expect short fragments, so design short amplicons (under about 150 bp). Formalin deamination produces artifactual C>T changes at low frequency; uracil-DNA glycosylase treatment and sensible variant thresholds keep them from being called as mutations.

Cell-free DNA pre-analytics

Cell-free DNA (cfDNA) circulates in plasma as short fragments (about 170 bp, one nucleosome’s worth). It’s so dilute that the steps before extraction decide the result. The danger is white cells lysing after the draw and flooding the plasma with genomic DNA, which dilutes the tumor or fetal signal.

• Tube: standard EDTA works only if processed within a few hours; cell-stabilizing tubes hold white cells intact for days, allowing shipping.
• Processing: separate plasma promptly, then spin twice (a second, harder spin removes remaining cells).
• Handling: avoid hemolysis and freeze-thaw cycles.

Inhibitors

Some substances survive extraction and block the polymerase even when nucleic acid is present ¹:

Defenses: thorough washes, a full dry step before elution, diluting the extract (inhibitors dilute too), and an internal control in every reaction, so an inhibited sample reads “invalid” rather than falsely “negative.”

What to take from this

Instruments run bind-wash-elute with magnetic silica beads, giving throughput, reproducibility, and fewer open transfers; cartridge and sample-to-answer systems stay closed from specimen to result. Match the method to the specimen: EDTA blood for DNA, short amplicons for crosslinked and deaminated FFPE DNA, quick processing or stabilizing tubes and a double spin for cell-free DNA. Avoid heparin, wash and dry well, and use internal controls to catch inhibition.