International Screening Limits (ISLs), International Residue Limits (IRLs) and Asian Screening Limits (ASLs) have the purpose to ensure that racing jurisdiction screen therapeutic substances (those which aid in the recovery of the horse) at scientifically agreed and therapeutically active concentrations. In the best interest of our racing, the NHA has historically adopted and applied most of these limits from the International Federation of Horseracing Authorities (IFHA) and the Asian Racing Federation (ARF). Examples of screening limits which are not adopted are those for Phenylbutazone and its metabolite (a drug which is banned for use in this country) and anti-ulcer medications (these are allowed by the NHA to be used within racing).

There are substances which are present in the natural environment, in the horse naturally and / or which are contaminants. Many of these are formalised as International Residue Limits (IRL’s) and these are carefully considered and studied by the NHA for their validity and applicability in this country before these are adopted. It must be noted that Atropine and Scopolamine (drugs substances but potentially also very harmful substances) are not adopted as IRLs by the NHA as these are abnormally high in prevalence in this country within toxic plants which have been shown to at times significantly contaminate local horse feed and roughage.  

The NHA has recent evaluated scientific data for a few new such ISL, IRL and ASL screening limits and these will now be adopted within the laboratory screening of these particular prohibited substances. These are explained and discussed below. Also explained and discussed is how these new limits impact the NHA International Thresholds and the NHA Detection Time guidelines. All of these documents are now updated and are available on the NHA website. These updated documents are effective 15 November 2021.

Note that International Thresholds are effectively screening limits but with the requirement that full accurate quantification of concentration is required when these are exceeded. In contrast, with so-called Screening Limits the laboratories must just show that quality controls at these limits are exceeded during the specimen screening process.

Note that NHA Detection Times provide a measure of the last time period screening limits have been shown to still be exceeded during horse administration trials of the substance. It is recommended that Withdrawal Times are at least about 1.4 times the Detection Times for oral and intravenous administration and twice the Detection Times for intramuscular or topical administration.

An ISL for Procaine is adopted at 0.02 ng/ml in PLASMA and an ASL for Procaine at 20 ng/ml in URINE.

• The above urine and plasma screening limits are equivalent in their Detection Times.
• It is pertinently stated that the urine ASL limit corresponds to a detection time of 12 days “following the administration of procaine penicillin intramuscularly at 12 mg/kg for 5 days.”
• Note that this 12 day detection time is now reflected in the NHA Detection Time guideline.
• Note that as an intramuscularly administration the Withdrawal Time must be about twice the Detection Time.
• Note that Procaine Penicillin can now be used in horses with a shorter Withdrawal Time than previously recommended within the NHA Detection Times / Withdrawal Times.

An ISL for Xylazine is adopted at 10 ng/ml in PLASMA and an ISL for Xylazine at 10 ng/ml in URINE (screening for the 4´-Hydroxyxylazine metabolite)

• The above urine and plasma screening limits are equivalent in their detection time.
• In order to assist with racing compliant administration of the substance, a NHA Detection Time is now provided for this therapeutic substance.

An ISL for Dantrolene is adopted at 0.1 ng/ml PLASMA (screening for 5-Hydroxydantrolene) and 3 ng/ml URINE (screening for 5-Hydroxydantrolene in unhydrolysed urine).

• The above urine and plasma screening limits are equivalent in their detection time.
• In order to assist with safe administration of the substance, a NHA Detection Time is now provided for this therapeutic substance.

An ISL for Vedaprofen is adopted at 5 ng/ml PLASMA.

• In order to assist with safe administration of the substance, a NHA Detection Time is now provided for this therapeutic substance.

An IRL for Dimethyl sulfoxide (DMSO) is adopted at 1000 ng/ml in PLASMA and at 15 µg/ml in URINE).

• This was previously an International Threshold and the concentration remains unchanged at 15 µg/ml URINE and 1000 ng/ml (this is equivalent to 1 µg/ml) in PLASMA. The only change with this new International Residue Limit is in the fact that there is now not full accurate quantification required of the substance. Now its needs to only be shown by the screening laboratory that the IRL was in fact exceeded during initial screening.
• The published NHA Detection Time of 36 hours remains unchanged as the prosecution concentration remains unchanged.
• A general notice is now in place on the NHA website to clarify the use of this substance in the horse both as intravenous administration as well as being applied on the skin.

An IRL for Methylsulfonylmethane (MSM, dimethyl sulfone) is adopted at 1200 µg/ml in URINE (effective 15 November 2021).

• This substance in found in products and supplements which contain MSM.
• An NHA Detection Time of 96 hours is now provided following the administration of MSM to the horse.
• A general notice is now in place on the NHA website to clarify the use of MSM in the horse and in order to accentuate that it is also a metabolite of DMSO.
  

An IRL for Caffeine is adopted at 20 ng/ml PLASMA

• This PLASMA level is the time equivalent of the previously NHA adopted IRL of Caffeine at 50 ng/ml URINE. The same Detection Time will therefore apply when Caffeine is administered to the horse.

International Thresholds

In accordance with most recent decision and wording from the IFHA, within the text for the NHA International Thresholds it is now additionally confirmed that the specific gravity of a specimen will not be considered during substance quantification. To not consider the specific gravity has been the policy of the NHA Laboratory for many years. There is therefore no change in either the manner of screening or the screening sensitivity. 

In accordance with most recent decision and wording from the IFHA, within the International Thresholds it is now additionally specified that when Testosterone in the urine of a gelding exceeds the existing prosecution threshold, then it must be confirmed that the concentration of Epi-Testosterone is at least 5 times lower than that of the Testosterone. This condition is adopted by the NHA as it improves the reliability that positive findings are only declared on specimens when actual exogenous administration of Testosterone occurred in the horse.

NHA Detection Times

The list of listed detection times was amended to include a shorter (more attractive) period for Procaine (when administered as Procaine Penicillin). While new screening limits were introduced for Dantrolene, Methylsulfonylmethane (MSM), Vedaprofen and Xylazine, detection times are now also specified for these therapeutic substances. The updated detection time list now also additionally incorporates Butorphanol, Clanobutin, Dembrexine and Naproxen.