Consistency in the Assessment of Dried Blood Spot Specimen Size and Quality in U.K. Newborn Screening Laboratories

Abstract

In 2015, U.K. newborn screening (NBS) laboratory guidelines were introduced to standardize dried blood spot (DBS) specimen quality acceptance and specify a minimum acceptable DBS diameter of ≥7 mm. The UK 'acceptable' avoidable repeat rate (AVRR) is ≤2%. To assess inter-laboratory variability in specimen acceptance/rejection, two sets of colored scanned images (n = 40/set) of both good and poor-quality DBS specimens were distributed to all 16 U.K. NBS laboratories for evaluation as part of an external quality assurance (EQA) assessment. The mean (range) number of specimens rejected in the first EQA distribution was 7 (1-16) and in the second EQA distribution was 7 (0-16), demonstrating that adherence to the 2015 guidelines was highly variable. A new minimum standard for DBS size of ≥8 mm (to enable a minimum of six sub-punches from two DBS) was discussed. NBS laboratories undertook a prospective audit and demonstrated that using ≥8 mm as the minimum acceptable DBS diameter would increase the AVRR from 2.1% (range 0.55% to 5.5%) to 7.8% (range 0.55% to 22.7%). A significant inverse association between the number of specimens rejected in the DBS EQA distributions and the predicted AVVR (using ≥8 mm minimum standard) was observed (r = -0.734, p = 0.003). Before implementing more stringent standards, the impact of a standard operating procedure (SOP) designed to enable a standardized approach of visual assessment and using the existing ≥7 mm diameter (to enable a minimum of four sub-punches from two DBS) as the minimum standard was assessed in a retrospective audit. Implementation of the SOP and using the ≥7 mm DBS diameter would increase the AVRR from 2.3% (range 0.63% to 5.3%) to 6.5% (range 4.3% to 20.9%). The results demonstrate that there is inconsistency in applying the acceptance/rejection criteria, and that a low AVVR is not an indication of good-quality specimens being received into laboratories. Further work is underway to introduce and maintain standards without increasing the AVRR to unacceptable levels.

Keywords: avoidable repeat rate; blood spot quality; dried blood spots; external quality assessment; filter paper; specimen collection; specimen collection device.

Figure 1

The number of DBS specimens rejected by each laboratory in (a) EQA distribution 1 and (b) EQA distribution 2. Dotted lines show the mean number of specimens rejected. A total of 40 DBS images were disseminated for each distribution. The laboratory numbers are the same in both distributions. NB—Laboratory 11 did not return results in EQA distribution 2.

Figure 2

Examples of DBS specimen images included in the two separate EQA distributions. Image 1 shows a good-quality specimen (0/15 labs rejected this specimen), image 2—multi-spotted (11/15 labs rejected), image 3—poor quality/uneven saturation of blood (8/14 labs rejected), image 4—blood applied to both sides of filter paper (9/14 labs rejected), image 5—wet specimen placed in glassine envelope/compressed (10/14 labs rejected), image 6—Insufficient/too small (9/14 labs rejected), image 7—excess blood/layering (2/15 labs rejected), image 8; a compressed specimen (6/15 labs rejected).

Figure 3

The relationship between the mean number of specimens rejected in the DBS quality assessment distributions 1 and 2 and the UK AVVR (%) in the U.K. during the period of the study (2018–2019); r = 0.194, p = 0.506.

Figure 4

Impact of implementing the ≥8 mm minimum DBS specimen diameter rejection criteria on the AVRR in the 16 U.K. NBS Laboratories. The AVRR is calculated from DBS specimens routinely received into the individual laboratories. The laboratory numbers are the same in all assessments.

Figure 5

The relationship between the mean number of specimens rejected in the DBS quality EQA assessment distributions 1 and 2 with (a) the predicted AVVR (%) if the minimum diameter of ≥8 mm was introduced; r = −0.734, p = 0.003 and (b) the predicted AVVR (%) if the new visual guide SOP was introduced and using the existing ≥7 mm as the minimum acceptable standard; r = −0.651, p = 0.01.

Figure 6

Breakdown of DBS specimens rejected in the U.K. during the period 2018–2021. The number of babies screened during 2018/2019, 2019/2020 and 2020/2021 was 741,577, 723,295 and 689,794 respectively. NB—these figures are based upon the 2015 rejection criteria [Supplementary File S1].

Figure 7

Impact of implementing the visual guide SOP based upon the 2015 criteria and using ≥7 mm as the minimum acceptable diameter on the AVRR in the U.K. NBS laboratories. The laboratory numbers are the same in all assessments.