Sample analyser-change report, CD4

paired CD4 counts: 1,885
results changed category, unadjusted: 535
expected from repeat testing alone: 238
of the excess caught on unseen data: 66%

The report as delivered

A print-ready PDF: executive summary and recommendation, figures at each clinical threshold, the re-test band, a note for clinicians, method and source.

Download the PDF

1. What changed

1,885 CD4 counts were measured on both platforms. On average the new platform reads about 14% lower than the old one. The comparison line has a slope of 0.87, so the gap grows with the count.

Sample

Fig. 1. Where the new platform reads at each CD4 clinical threshold. The comparison line (slope 0.87) sits below the line of no change, so the new platform reads lower, by about 14% on average across the paired counts.

Source Coetzee LM, Glencross DK (2017). PLoS One. doi:10.1371/journal.pone.0187456 (opens in a new tab); PMC5669480 (opens in a new tab). Licence: CC BY 4.0 (opens in a new tab). Re-analysed by NakedSignal.

Show the numbers

CD4 sample comparison line
Measure	Value
Comparison line slope (Passing-Bablok)	0.867
Slope, 95% interval	0.857 to 0.878
Comparison line intercept (cells/µL)	-1.2
Mean difference, new minus old, as % of old	-14.284
Clinical thresholds (cells/µL)	100, 200, 350, 500

2. Bias at each clinical threshold

Where the new platform reads at each threshold, read off the comparison line.

Bias at each CD4 clinical threshold, from the comparison line
Threshold (cells/µL)	New platform reads (cells/µL)	Difference (cells/µL)	Difference (%)
100	85.5	−14.5	−14.5
200	172.1	−27.9	−13.9
350	302.1	−47.9	−13.7
500	432.1	−67.9	−13.6

3. Which results cross a clinical threshold

The clinical thresholds are 100, 200, 350, 500 cells/µL. Reported without adjustment, 535 results fall in a different CD4 category on the new platform. Repeat testing on one unchanged platform would move about 238 results across the same thresholds at a default repeat-test imprecision; if that default fits this assay, the rest is what the change itself did.

A laboratory's own report also breaks the count down by threshold and by direction, old category to new.

Sample

All four clinical thresholds (100, 200, 350, 500 cells/µL)

Crossed between the two systems: 535

Expected from repeat testing alone: about 238

Fig. 2. Sample built from public data: 1,885 paired CD4 counts from an open-access deposit, one platform replaced by another. Reported without adjustment, 535 results change category against about 238 expected from repeat testing alone. Flagging the results that land in a narrow band around each threshold for re-test, and converting the rest with the comparison line, catches 66% of that excess on data the band was not fitted to, while re-testing 15% of results; the comparison line alone catches 46%.

Show the numbers

CD4 sample: results crossing a clinical threshold
Measure	Value
Paired CD4 counts	1,885
Clinical thresholds (cells/µL)	100, 200, 350, 500
Results that changed CD4 category, unadjusted	535
Expected to change from repeat testing alone	238.49
Excess caught by comparison line and re-test band, unseen data (%)	66
Results re-tested, unseen data, median (%)	15
Repeat-test imprecision	Default value

4. Review band during the transition

Each new result is corrected to the old scale with the comparison line. Results that land inside a narrow band around a threshold are close enough that the change could have moved them; they are candidates for review, and the laboratory decides whether to repeat them. For this dataset the band is a fixed factor either side of each threshold: it reaches 5.51% above it and slightly less than that below it.

Tested on data the band was not fitted to, the comparison line and band together catch 66% of the excess crossings while re-testing 15% of results. The comparison line alone catches 46%.

excess caught, line and band: 66%
excess caught, line alone: 46%
of results re-tested: 15%

5. A note for requesting clinicians

The one-page note, written from the same numbers. A laboratory adapts it to its own words.

SampleCD4 counts: change of measuring platform

The laboratory is moving CD4 counts to a new platform. On the same samples, the new platform reads about 14% lower than the old one, and the gap grows with the count.

Without adjustment, some results near the thresholds we report against (100, 200, 350, 500 cells/µL) would fall in a different category than on the old platform. During the transition the laboratory corrects new results to the old scale and reviews results that fall close to a threshold, so fewer categories change because of the platform alone.

If a patient's CD4 category differs from an earlier result, the platform change is one possible reason. Please contact the laboratory before acting on a change of category alone.

Every part of a laboratory's report

A laboratory's own report is built from its own paired results, thresholds and repeat-test imprecision, and covers the first six parts listed below. For a network, the seventh is scoped on request.

The comparison line, with its interval.
How the new analyser's results relate to the old one's across the whole range you measured.
Bias, with limits of agreement.
How far apart the two analysers read on the same sample, and how much that varies.
Bias at each of your own decision limits.
Not an average: the difference at the concentrations where your clinicians act.
Patients across each limit, in each direction.
How many results changed category, set against how many repeat testing alone would have moved.
A re-test band for the transition.
A narrow range around each limit inside which a new result is repeated, sized to a budget you choose.
A one-page note for requesting clinicians.
What changed and what it means for their patients, in their words rather than ours.
For networks, whether sites report interchangeably.
On request, the same analysis across sites, at your limits, showing which site moves which patients. Scoped per network; not part of the standard program yet.

Source and notes

The thresholds at 350 and 500 cells/µL are former cut-offs for starting treatment, kept because the study's authors categorised CD4 counts at them.
This sample uses a default repeat-test imprecision; a laboratory's report uses its own.

Changing a platform soon?

Request a first site report