Cystic fibrosis

CF diagnoses in adults requires high level of clinical suspicion

About one in 20 people with CF are diagnosed as adults, according to data from the Australian Cystic Fibrosis Data Registry.

The data on adult diagnoses, published in The MJA [link here], suggests the disease in adults presents with milder symptoms than classical CF.

“High clinical suspicion is consequently needed by specialists and general practitioners to expedite its diagnosis,” the study authors said.

The Research Letter, led by a team from the Royal Prince Alfred Hospital in Sydney, described characteristics of 146 adults diagnosed with confirmed CF and listed in the registry between January 2000 and December 2019.

The adults represented 4.5% of all 3,446 registrations. One third (31%) of adults were diagnosed between 18 and 30 years, 46% between 31 and 50 years, and 20% over 50 years of age.

The study found 31% of adults for whom the CF diagnosis was suggested by clinical features presented with non-respiratory symptoms such as gastrointestinal disease and infertility.

“Other features reported at diagnosis in some cases included chronic sinus disease and family history of CF,” it said.

The data showed people diagnosed with CF as adults were generally well nourished based on BMI and had mildly reduced lung function.

“While evidence of Pseudomonas colonisation was infrequent at diagnosis (9 of 109, 8%), it was later identified in 35 of 62 patients (56%),” the study said.

“Our findings indicate that CF diagnosed in adults may manifest with milder symptoms across a range of organ systems than classical CF.”

It found 70% of the adults with CF were heterozygous for F508del mutations and a further 4% were homozygous. One quarter (25%) had other mutations.

“Finally, although the disease course was milder than for infants and children diagnosed with CF, lung transplant-free survival was still shorter than for people without CF.”

Tobramycin use in CF

Meanwhile, an online survey of IV tobramycin use for respiratory exacerbations of CF has found a lack of consistency with dosing and infusion times, therapeutic drug monitoring (TDM) for dose adjustment, and blood sampling methods.

The survey circulated via CF specialist groups in North America (US and Canada), the UK and Ireland, and Australia and New Zealand, found the most common initial dose was 10 mg/kg/day (74%) but doses ranged from 3.5 mg/kg/day to 12 mg/kg/day.

Infusion practices also varied with 21-60 min the most frequent duration (93%), but some administered as a 5-10 min bolus and other infusions lasting >60 to 90 min.

“There is a lack of evidence to show one infusion rate is superior; however, when targeting Cmax/MIC, a more rapid infusion would optimise Cmax and theoretically improve the efficacy of this concentration-dependent antibiotic,” the investigators said.

They said computer programs (44.4%) and first principles (22.2%) were the most common TDM approaches in Australia and New Zealand centres and in North America (28% and 51% respectively) while all UK and Ireland sites used monitoring of Ctrough only.

Venous blood sampling was the most common method of blood collection both within hospitals (70%) and out-of-hospital settings (71%).

It noted that the evolution of TDM-based dose adjustment strategies has also been accompanied by advancements in blood sample collection methods which use either alternate matrices or sampling techniques.

“The most promising of these is capillary blood sampling, which is less invasive than traditional venepuncture and more readily enables collection of samples in non-acute care settings.”

Given some of the variations, the investigators called for an update to national guidelines such as the Cystic Fibrosis Pulmonary Guideline, with clear recommendations on TDM approaches and dosing recommendations.

The study, led by SA pharmacist Rebecca Larcombe, was published in the European Journal of Hospital Pharmacy [link here].

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