Processing the sample in the laboratory

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Sources of error

Procedures for processing cervical samples are described elsewhere. There are many diagnostic errors that can occur due to poor processing techniques in the laboratory:

Clerical procedures for receiving the samples

• Inconsistencies between information on the request form and sample.
• Misspelling of name or incorrect date of birth leading to screening history not being accessed and linked.

Staining procedures

• Poor quality staining and fixation is the single most likely cause of errors of reporting conventional cervical smears. 
• Cell morphology may be distorted or obscured by ‘cornflake’ artefact caused by air trapped if there is a delay between removing the slides from xylene and applying the mounting medium.
• Using out-of-date reagents will lead to pale nuclear staining and loss of nuclear/cytoplasmic contrast.
• Retrogressive Papanicoloau stains are more ‘at risk’ than progressive stains for hypochromic staining leading to ‘pale dyskaryosis’.
• If the acidity of the water and acid water is not controlled correctly, the nuclear outline and chromatin structure will be poorly displayed. 

Methods for QA and QC of processing the sample

• Laboratory supervisor should carry out regular checks of accuracy of computer data entry (QC).
• Standard operating procedures (SOPs) must be observed by clerical staff entering data (QC).
• Feedback to sample takers concerning (i) apparently incorrect, incomplete or illegible clinical data, (ii) mismatches between sample and request form and (iii) poorly spread or fixed conventional smears (QC).
• Where possible, introduce a barcode or automated processing system for labelling request form and slide (QC).
• Introduce a standard operating procedure (SOP) for staining and make sure the protocols are followed exactly (QC).
• Keep a record of changes to stains and when they are filtered and topped up (QC).
• Select a coverslip that will cover as much as possible of the cellular material on the slide (QC).
• The laboratory supervisor should check random slides for intensity of stain, nuclear/ cytoplasmic contrast, dehydration and clarity of mounting medium on a daily basis (QC).
• Take part in regional or national technical EQA schemes (QA).
• Laboratory accreditation to ensure compliance with national standards (QA)