JISCMail - ACB-CLIN-CHEM-GEN Archives

I agree that anytime a specimen is rejected for analysis, for not meeting pre-analytical requirements, there is the danger of missing or delaying the diagnosis of a medical emergency.

In the current regulatory environment we are required to have all our procedures documented and to audit them to ensure that they are complied with.

With ammonia there is much published information about the effect of delayed separation on the accuracy of the result but no consensus, that I could find, on the maximum duration acceptable between collection of the sample and separation of the plasma from the cells. Many commentators state that the sample should be placed on ice immediately and then centrifuged at 4 ºC but fail to state how much time can elapse between placing the sample on ice and centrifuging.

I personally am happy to process a sample that has arrived in the lab in a "reasonable" time-frame and to add a free text comment to highlight the effect that delayed separation will have on the result. For less experienced members of staff you have to have an SOP which clarifies where the line is drawn or we risk processing samples which are several hours old and sending them out with an automatic comment.

It would be appreciated if Dr Henderson could share the maximum time allowed to elapse between sample collection and separation that he has documented in his lab. I know from attending a BIMDG meeting that he has much experience in this topic.

Regards,

Gavin Murdock

On 29 April 2016 at 09:13, Henderson Mick (LEEDS TEACHING HOSPITALS NHS TRUST) <[log in to unmask]> wrote:

Dear Colleagues,
can I urge extreme caution when enforcing policies that result in the rejection of samples for ammonia analysis.
At the very least such a policy must be discussed with the clinical users of the service.
Hyerammonaemia, at levels above 350umol/L, is likely to be a medical emergency with potentially irreversible consequences for brain function and may be lethal.
There is a balance to be struck between the obligation to report accurate results with the clinical imperative to identify dangerously aberrant biochemistry.
Clinical teams are much more likely to quickly repeat a blood test when presented with a high result on a 'delayed' sample.3
Such results must, of course be accompanied by an appropriate warning and must be telephoned.
We are aware of clinical catastrophes caused by the rejection of samples that were then not repeated for 24 hours, for a variety of reasons.
If a patient has an inborn error of metabolism the metabolic crisis can deepen rapidly.

Best Wishes,

Mick Henderson

Dr MJ Henderson
Laboratory Director
Willink Biochemical Genetics Unit Biochemical Genetics
Manchester Centre for Genomic Medicine St James’s University Hospital
Central Manchester University Hospitals Leeds Teaching Hospitals Trust
Saint Mary's Hospital Beckett Street
Oxford Road, Manchester. M13 9WL Leeds. LS9 7TF
Phone:0161 701 2137 Phone:0113 206 4107
________________________________________
From: Clinical biochemistry discussion list [[log in to unmask]] On Behalf Of Gavin Murdock [[log in to unmask]]
Sent: 28 April 2016 22:13
To: [log in to unmask]
Subject: Re: Ammonia

1. Ammonia in plasma exists in equilibrium with the ammonium anion. Ammonia is also present in the atmosphere. When the sample is collected there is the possibility of the equilibrium being disturbed by the addition or loss of ammonia to the air space.

To mitigate against this affect we only analyse samples that arrive in the lab within 15 minutes of collection. You could reject grossly under-filled specimens but my experience is that such specimens are likely to be haemolysed and so would be rejected on that basis - ammonia is present in red cells at ~3 times the concentration of plasma.

2. The key step is to separate the plasma from the cells. Once that has happened ammonia still increases due to deamination reactions but at a slower rate. Low temperatures retard these reactions and so increase the stability.

We allow promptly separated plasma to be refrigerated if it can be analysed within 2 hours, otherwise it should be frozen.

Gavin

On 28 April 2016 at 14:00, Pullan Nicola (ROYAL UNITED HOSPITALS BATH NHS FOUNDATION TRUST) <[log in to unmask]<mailto:[log in to unmask]>> wrote:
Hello,

Can anyone help with the following?

1. Our pack insert (Cobas) states that EDTA blood tubes should be full for ammonia analysis. We make no provision for this currently and don't bounce requests if the bottles are under-filled. Any opinion?

2. If samples are separated from the cells and aliquoted into a sealed false bottomed tube for analysis at a later point e.g. if analyser out of action, any idea of stability?

Many thanks for any answers in advance.

Best wishes,

Nicola
Nicola Pullan
Principal Clinical Biochemist
Royal United Hospitals Bath NHS Foundation Trust
Combe Park, Bath, BA1 3NG
Dir Line: 01225 824711
Visit our website at: www.ruh.nhs.uk/pathology<http://www.ruh.nhs.uk/pathology>

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