Repeat Samples & Limitations of Testing

Key factors which are known to affect performance of the screening tests.

Working in conjunction with midwives and other health professionals we aim to reduce the number of 'avoidable repeats' as far as possible. Patient safety is our utmost concern, and if we request a repeat sample, it is to ensure we get the correct result, on the right baby, so each baby who has one of the diseases we screen for, can access appropriate treatment in a timely manner.

Avoidable repeats: Key factors known to affect performance of the screening tests.

1. Problems arising before sample collection:

  • Baby too young (e.g. day 4)
  • Expired card
  • Details missing or inaccurate:
    • No NHS number / Date of birth / Date of sample

TIP: Ask the parents to check the card (if necessary make amendments to the label and/or card).

 

2. Problems arising during sample collection:

  • Insufficient - blood spots too small
  • Insufficient - Inappropriate application of blood e.g. not soaked through, applied both sides or multi-spotted.
  • Compression of sample
  • Contamination of sample (e.g. with faeces).
 

TIPS:

You can ask for the baby to be held in a different position to make it easier for you to take a good quality sample.
Prick the heel and wait for the blood to flow.

Fill each circle on the card with a single drop of blood - check the blood has soaked through to the back of the card.

Take your time - If the baby is not blooding it may be necessary to prick the heel again. Do not squeeze.
 

3. Problems arising after sample collection:

  • Sample contamination (e.g. with faeces)
  • Water damage
  • Arrival in laboratory >14 days after collection
  • Analytical problem / Unsatisfactory analysis.
 

TIP: Samples sent by courier arrive in the lab more quickly.

 

Resources

 

Unavoidable repeats

Occasionally, we will contact the midwife and ask for a repeat sample for one of the following reasons. In these cases, the original sample was collected correctly, but the results for one or more of the tests was borderline or inconclusive.

  • Slightly raised TSH result - Repeat should be collected 7 -10 days after the initial sample.
  • Inconclusive CF screen - Repeat should be collected between 21 - 28 days of age.
 

Other factors known to affect the reliability of the screening tests

  • Thalassaemia trait is not excluded.
  • The Sickle Cell Screen is only valid if baby not transfused.
  • IRT is not always abnormal in CF patients with meconium ileus; therefore the screening result may be falsely normal.
  • IRT is an unreliable indicator of CF in babies >8 weeks of age. Therefore the test will not be performed in samples collected in this age group.
  • Some babies with MCADD, MSUD or IVA may become symptomatic in the first few days of life, before the screening test has been carried out. In this case the clinician looking after the baby should send clinical samples to their usual diagnostic laboratory. 
  • CHT screening tests for raised TSH only; therefore if hypothyroidism occurs secondary to pituitary disease, the screening result may be falsely normal.
  • Newborn bloodspot screening is only offered to babies up to 1 year of age. In older babies, other tests are available to doctors through clinical pathways for babies/children with suggestive symptoms.

This list is not exhaustive and it is important to remember that these tests are screening tests; no screening test is 100% reliable. Further information about the limitations of newborn screening tests can be found in the following laboratory handbooks: