Venous and capillary collection: a correlation study

Abstract

Capillary collection removes the phlebotomist from the draw, making timed and at-home collection practical. None of that matters unless the result is the same. Thirty volunteers were enrolled and twenty paired specimens carried into a Passing-Bablok comparison of capillary against venous collection. Agreement met the acceptance criteria across the analytes evaluated. Dihydrotestosterone did not, and is reported separately rather than folded into the summary.

Matrices evaluated: Venous serum, Tasso capillary serum

Capillary collection removes the phlebotomist from the draw. A medical assistant can perform it, a properly trained patient can perform it at home, and a timed specimen becomes practical rather than logistically difficult.

None of that matters unless the result is the same. Clinicore ran a controlled comparison to establish whether it is.

Design

Thirty healthy volunteers were enrolled — twenty female and ten male. Capillary specimens were collected from the non-dominant hand, immediately followed by a venous draw from the opposite arm, so both specimens represent the same moment. Twenty paired specimens were carried into the comparison.

Each analyte was evaluated by Passing-Bablok regression, with venous concentration on the x-axis and capillary on the y-axis. Passing-Bablok was chosen because it assumes measurement error in both methods rather than treating one as a reference standard — the correct assumption when comparing two collection routes for the same assay.

Acceptance required a correlation of r ≥ 0.96, with slope and intercept falling within the 95% confidence interval.

Run at enhanced sensitivity, deliberately

The study was performed at the method’s enhanced sensitivity configuration rather than at production reportable limits. The question was whether capillary collection introduces a difference at all, and the most sensitive configuration is the one most likely to expose one. Finding no proportional bias under those conditions is a stronger result than finding none at a less sensitive setting.

One consequence follows and is worth stating rather than explaining away later: several study minima sit below current reportable limits. The study predates go-live; production limits were set afterwards, backed off to concentrations reproducible at volume.

Results

Agreement met the acceptance criteria across the analytes evaluated.


Analytes evaluated	14
Paired specimens	20
Correlation	at or above the 0.96 acceptance threshold
Slope	0.94 – 1.09, clustering at 1.00
Slope 95% confidence interval	contains 1.00

That last line is the finding. A slope of exactly 1.00 would mean the two collection routes agree perfectly; a confidence interval containing 1.00 means any departure from perfect agreement is within measurement error rather than a real difference.

Per-analyte correlations, slopes, intercepts and confidence intervals are held in the validation record and available to ordering clinicians on request.

Dihydrotestosterone: the exception

DHT did not meet the acceptance criteria and is not included in the summary above.

Its slope was 1.57 against a median near 1.00 across the other analytes, and it was evaluable in 15 of the 20 pairs rather than all 20 — five specimens fell below the reportable limit then in force and were excluded.

Two reasons this is stated rather than omitted:

A reader comparing this page against older marketing material will find the difference. Earlier material reported that the slopes of all hormones were near 1.0, and included DHT in the same table that shows 1.57. Both statements cannot be true, and the table is the one to believe.

The finding does not extend to DHT. Where DHT is the clinical question, capillary equivalence is not established by this study, and Clinicore says so rather than letting a summary statistic imply otherwise.

The DHT reportable limit has since been extended downward — it is now 10.125 ng/dL, against the 15.00 ng/dL in force when this study ran. See the DHT analyte page for the current figure. Restating this study against today’s limit would misdescribe which specimens were excluded and why.

What this establishes

Capillary collection produces results equivalent to venous collection across the measured range for the analytes evaluated — which is the basis on which Clinicore offers the Tasso capillary device for endocrine testing.

It does not establish equivalence for DHT, and it does not extend to analytes outside the panel studied.

This study was conducted beyond what regulatory validation requires.

Per-analyte correlation data is available to ordering clinicians on request.