Sperm-negative high vaginal swab with a strong PSA result
A 24-year-old woman reports an assault 96 hours earlier. The SANE collects a high vaginal swab, a cervical swab, and an external vulvar swab. The high vaginal swab is PSA-positive at a 1:1,000 dilution, but two wet-mount examinations by an experienced analyst show no intact spermatozoa. The cervical swab is PSA-negative and yields abundant squamous epithelium. The vulvar swab is PSA-positive but carries visible lubricant. Laboratory protocol performs a two-step differential extraction on all semen-positive swabs and directly lyses external swabs. The case is due to the prosecutor in three weeks, and the laboratory has only four differential columns left in the lot.
1. Rank the three swabs for DNA analysis and explain how the sperm-negative microscopy changes, or does not change, that ranking.
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Rank the high vaginal swab first, the vulvar swab second, and the cervical swab third.
The high vaginal swab is the only internal sample with a strong PSA signal at high dilution, which indicates seminal fluid deposition even though no sperm were seen. Absence of sperm does not exclude semen: the donor may be vasectomized, azoospermic, or oligospermic, and PSA detects seminal fluid rather than sperm. Downgrading this swab because of the microscopy would be an error.
The vulvar swab is worth extracting because it samples an external deposit that may contain DNA from the assailant's skin or semen, but the lubricant may inhibit amplification and the swab will carry heavy female DNA.
The cervical swab is lowest priority: it is PSA-negative and dominated by squamous epithelium, so it is more useful for confirming the complainant's own profile than for detecting the assailant.
2. Should the laboratory run its standard differential extraction on the high vaginal swab? Justify your answer given the column shortage and the microscopy result.
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No - not as a routine two-step extraction, and not with a scarce column.
The sperm fraction of a differential extraction is designed to capture sperm heads. With no sperm seen, that fraction is likely to be empty or nearly so, while the male DNA present as seminal fluid may be soluble and end up in the epithelial (supernatant) fraction. Running the differential therefore risks spending a column and still recovering the male DNA only where the protocol does not expect it.
A better plan is to split the swab: directly lyse a portion for total DNA, and retain the remainder. If the laboratory does run a differential, the epithelial fraction must be retained and typed, not discarded.
Reserve the four remaining columns for samples in which sperm are actually observed. The column shortage is an operational constraint, not a reason to skip a scientifically necessary step; here the step is not necessary.
3. What additional typing strategy might you consider, and what interpretive limitation must be disclosed if it is used?
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Consider Y-STR analysis on the high vaginal extract if autosomal typing yields an uninterpretable female-male mixture.
Y-STRs amplify male-lineage DNA even when female DNA is in large excess, and they do not require sperm, so a vasectomized or azoospermic donor is still detectable. This directly addresses the risk that the autosomal profile is swamped by complainant DNA.
The limitations must be stated in the report:
- A Y-STR haplotype is shared by paternal relatives and is not individualizing; it supports exclusion far better than inclusion.
- If more than one male contributed, the result is a Y-haplotype mixture that may be uninterpretable.
- A Y-STR result cannot be searched against an autosomal offender index, and a match requires confirmation with autosomal testing.
- If no male DNA is present, a null result does not mean no contact occurred.
Autosomal typing remains the primary analysis; Y-STR is a supplement, not a substitute.