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PA7.1 | Diagnostic Cytology — SDL Guide (Part 3)

Liquid-Based Cytology (LBC) and Automation

Liquid-Based Cytology and Automated Screening

Liquid-based cytology (LBC) is a refinement of the conventional smear developed to reduce artefacts and improve cell preservation.

Technique:
1. Instead of smearing directly onto a slide, the brush is rinsed into a vial of preservative liquid (ThinPrep™ uses methanol-based PreservCyt; SurePath™ uses an ethanol-based fixative).
2. In the laboratory, cells are dispersed, contaminants (blood, mucus) are reduced by filtration or density gradient centrifugation, and a thin monolayer is deposited onto a glass slide.

Advantages over conventional smear:
• Fewer obscuring elements (blood, mucus, inflammatory cells)
• Cells preserved in near-native round morphology (not distorted by smearing)
• Residual liquid can be used for HPV molecular testing, immunocytochemistry, or cell-block preparation
• Standardised thin monolayer — easier for automated screening
• Longer shelf life

Automated screening (AI-assisted):
• ThinPrep Imaging System / BD FocalPoint — computer-assisted prescreening that ranks fields of view by probability of abnormality
• Cytotechnician reviews the flagged fields; reduces false-negatives from fatigue
• Artificial intelligence-based screening tools are now being validated for primary autonomous screening in high-volume national programmes

Limitation: LBC is more expensive than conventional smear; in resource-limited settings, conventional Pap smear remains the standard.

Diagnostic Role of Cytology — Applications, Screening, and When Biopsy is Still Needed

Cytology serves three overlapping clinical roles:

1. Screening (population-level)
• Cervical cancer: Pap smear / LBC — the paradigm of successful cancer screening; reduces mortality > 70% with organised programmes
• Lung cancer in high-risk smokers: sputum cytology (adjunct, lower sensitivity)
• Urothelial cancer surveillance: urine cytology for known bladder cancer follow-up

2. Diagnosis (individual patient)
• Palpable lump: FNAC → defines benign vs malignant, guides surgery
• Effusion: effusion cytology → detects metastatic deposits, lymphoma, mesothelioma
• Respiratory mass: bronchial brushing / BAL cytology
• Organ lesion (thyroid, liver, pancreas): image-guided FNAC

3. Monitoring and staging
• Repeat cytology to assess response to therapy (leukaemia, carcinoma in situ)
• CSF cytology for CNS involvement in leukaemia/lymphoma

Diagnostic Roles and Limits of Cytology

When histological biopsy is still required (cytology is NOT enough):

Situation	Reason histology is needed
Follicular thyroid neoplasm	Capsular invasion only detectable on intact section
Lymphoma subtyping	Architecture + flow cytometry + IHC on intact node
Soft-tissue tumour grading	Extensive necrosis assessment, mitotic count per HPF
Assess surgical margins	Requires en bloc excision section
Inconclusive FNAC (Bethesda III or IV)	Risk-stratify before definitive surgery

Remember: A positive cytology = cancer until proven otherwise. A negative cytology = low probability, but does NOT rule out malignancy (10-20% false-negative rate depending on site and technique). The triple test (clinical + imaging + cytology) reduces false-negatives in breast disease; all three must be concordant before proceeding conservatively.