Periprosthetic joint infection (PJI) remains one of the most serious complications after hip and knee arthroplasty. Registry-based analyses suggest an infection burden close to 1 % in contemporary arthroplasty practice, although rates may be higher in frailer populations: after hip hemiarthroplasty for femoral neck fracture, a recent systematic review reported a pooled deep infection rate of 2.14 %, rising to 2.64 % at 12 months (Springer et al., 2017; Silas et al., 2024). This burden is likely underestimated because debridement, antibiotics, and implant retention (DAIR) procedures are not consistently captured in national registries (Springer et al., 2017; Lenguerrand et al., 2018). PJI is now a leading cause of failure after total knee arthroplasty and an increasingly important cause of revision after total hip arthroplasty, accounting for 21.6 % of knee revisions and 18.2 % of hip revisions in a recent registry-based analysis (Koh et al., 2017; Hauer et al., 2024). Its absolute impact is expected to increase as arthroplasty volumes expand and treated populations become older and more comorbid (Kurtz et al., 2007).

Patients with PJI may require repeated procedures, prolonged antimicrobial treatment, and extended hospitalisation, and may experience persistent functional impairment. Survival is also adversely affected; early PJI after primary total knee arthroplasty has been associated with higher mortality than non-infected controls, reaching 15.7 % at 5 years and 38 % at 10 years (Thompson et al., 2022). More than 20 000 PJIs arising from primary arthroplasties performed in Europe in 2019 were estimated to account for EUR 346 million in reimbursement payments, and septic revision may cost up to three times more than aseptic revision (Alt et al., 2025).

Identifying host-related factors associated with poor outcome is therefore clinically relevant. Malnutrition and impaired immunonutritional status are increasingly recognised as potentially modifiable risk factors in arthroplasty, although reported prevalence varies widely according to the definition used (Cross et al., 2014; Almeida et al., 2024). A recent meta-analysis showed that malnutrition was associated with higher odds of surgical site infection and PJI after total joint arthroplasty (Chen and Chen, 2024). In established PJI, conventional nutritional biomarkers may also provide prognostic information; hypoalbuminaemia is common and has been associated with failure after both one-stage and two-stage revision (Green et al., 2023; Traverso et al., 2023; Li et al., 2024).

The controlling nutritional status (CONUT) score combines serum albumin, total lymphocyte count, and total cholesterol, and was originally developed as a hospital-based nutritional screening tool (de Ulíbarri et al., 2005). These three components are intended to reflect protein reserve, immune-nutritional status, and caloric depletion, respectively. In primary hip and knee arthroplasty, Nutritional Risk Screening 2002 (NRS-2002) and CONUT have been linked to postoperative infectious complications and quality-of-life domains, while admission CONUT has also been associated with 1-year PJI risk after total joint arthroplasty (De Mauro et al., 2026; Guo et al., 2026). In established PJI, moderate-to-severe CONUT has been associated with failure after two-stage exchange (Li et al., 2022). However, the prognostic value of CONUT as a composite host-status marker in acute PJI remains insufficiently defined.

The aim of this study was to determine the prevalence of high CONUT in patients surgically treated for acute hip or knee PJI and to evaluate its association with treatment failure and mortality alongside conventional inflammatory biomarkers and established host-assessment tools.

High preoperative CONUT identified patients with worse crude and time-to-event outcomes after surgery for acute PJI and remained independently associated with failure, total failure, and overall mortality after adjustment. The sensitivity analysis using continuous CONUT showed a similar association with overall mortality, consistent with previous evidence of increasing PJI-related risk at higher scores (Li et al., 2022; Guo et al., 2026). These findings support CONUT as a marker of preoperative host vulnerability, although the limited number of events warrants cautious interpretation.

Previous studies similarly linked hypoalbuminaemia with poor outcomes in established PJI. After one-stage septic revision, Traverso et al. (2023) found hypoalbuminaemia to be the only independent predictor of failure (OR 5.64), whereas total lymphocyte count and their combined malnutrition definition were not associated with outcome. Green et al. (2023) reported higher failure after first-stage resection in patients with albumin < 3.5 g dL⁻¹ (adjusted OR 2.50), and Li et al. (2024) found that hypoalbuminaemia at both the first stage and reimplantation was associated with failure, with worse outcomes when it persisted until reimplantation.

In our cohort, CRP, CAR, and NLR did not show consistent associations across outcomes; Hong et al. (2019) similarly found no association between preoperative CAR and failure after two-stage exchange. Although this does not diminish their diagnostic or monitoring value, isolated inflammatory indices may provide a less consistent prognostic signal than composite host-response measures (Ali et al., 2024). High-CONUT patients more frequently had Staphylococcus aureus infection and positive blood cultures, whereas coagulase-negative staphylococci were more common in the low-CONUT group. The association between S. aureus and positive blood cultures was consistent with a more systemic infection phenotype. This accords with Campbell et al. (2022), who linked low albumin with culture-confirmed S. aureus musculoskeletal infection; and Li et al. (2024), who associated hypoalbuminaemia in chronic PJI with potentially more virulent organisms, including S. aureus. Hypoalbuminaemia and high CONUT may therefore reflect both host vulnerability and the systemic response to a more invasive infection phenotype, although this interpretation remains exploratory and does not establish causality.

Importantly, CONUT should not be interpreted as a formal diagnosis of malnutrition, particularly in acute PJI. According to the Global Leadership Initiative on Malnutrition (GLIM) framework, malnutrition requires at least one phenotypic and one aetiological criterion, rather than serum-based abnormalities alone (Cederholm et al., 2019). The updated European Society for Clinical Nutrition and Metabolism (ESPEN) guidance makes a similar distinction: albumin is prognostic and may identify patients at increased metabolic risk, but it should not be considered a nutritional marker per se (Weimann et al., 2025).

This distinction is especially relevant in our cohort because CONUT components were measured after symptom onset and close to surgery. In acute infection, systemic inflammation, bacteraemia, metabolic stress, and catabolic activation may impair the host's ability to maintain normal circulating albumin, lymphocyte, and cholesterol levels. Hypoalbuminaemia may reflect the hepatic acute-phase response and reduced albumin synthesis during systemic inflammation (Beyer et al., 2022). Lymphopenia may accompany immune dysregulation and bacteraemia, and total lymphocyte count may correlate poorly with nutritional status in older adults (de Jager et al., 2010; Kuzuya et al., 2005; Wang et al., 2024). Hypocholesterolaemia has also been linked to sepsis severity and mortality, probably reflecting the interaction between inflammation, lipid metabolism, and host response (Taylor et al., 2024). In acute PJI, CONUT may therefore be better understood as a composite host-response marker integrating nutritional and physiological reserve, inflammation, immune dysfunction, and catabolic stress. Without pre-infection measurements, however, we cannot distinguish pre-existing vulnerability from infection-induced deterioration.

Diabetes mellitus was independently associated with all three outcomes, consistent with DAIR literature identifying diabetes among relevant comorbidities (Abbaszadeh et al., 2026). Lin et al. (2022) reported higher reinfection after two-stage revision with HbA1c ≥ 7 %, although no definitive threshold was established. This association is biologically plausible because hyperglycaemia impairs neutrophil chemotaxis, phagocytosis, and bactericidal activity (Jafar et al., 2016). Mortality was also associated with age and fracture/revision indication, indicating that CONUT should complement rather than replace established PJI-specific host and complexity scores such as JS-BACH by adding an objective laboratory-based dimension of host status.

Strengths included the use of EBJIS diagnostic criteria and structured MSIS-ORT outcome assessment. This is relevant because a lack of standardisation in PJI outcome reporting remains a major limitation, and different definitions can substantially alter reported success rates (Borsinger et al., 2021; Tarabichi et al., 2025). The MSIS-ORT allowed transparent reporting of mutually exclusive outcome categories and helped to distinguish failure, total failure, and overall mortality.

The main limitation of this study was the exclusion of otherwise eligible patients because one or more laboratory components required to calculate CONUT were unavailable, which may have introduced selection bias. In our centre, albumin and total cholesterol are usually included in standard morning admission or preoperative laboratory profiles but are not consistently requested in urgent emergency department blood tests. This probably explains why CONUT components were missing in a substantial proportion of patients. However, included patients and those excluded because of missing CONUT components showed no significant differences in baseline demographic, clinical, host-related, inflammatory, microbiological, or blood culture characteristics, mitigating but not eliminating this risk. Other limitations include the retrospective single-centre design, modest sample size, limited number of events, and residual confounding. Surgical treatment was heterogeneous and non-randomised, reflecting routine clinical decision-making; therefore, treatment-specific comparisons should be considered descriptive rather than causal.

High preoperative CONUT was associated with failure, total failure, and overall mortality after surgery for acute hip and knee PJI. As a simple and widely available composite host-status marker, CONUT may support preoperative risk stratification in this high-risk population. However, in acute PJI, CONUT should not be interpreted solely as a nutritional marker because its components may be influenced by systemic inflammation, bacteraemia, and infection severity. Larger multi-centre studies should validate these findings and determine whether targeted optimisation strategies can modify these outcomes.