When Resistant Schizophrenia Isn’t Schizophrenia: Reassessment Reveals Hidden Diagnostic Divide in Clozapine Care

A striking finding has emerged from one of the largest real-world examinations of treatment-resistant schizophrenia referrals: many patients sent to specialist clinics because their psychosis seemed immune to standard antipsychotics do not, in fact, carry a diagnosis of schizophrenia at all. A retrospective study of 392 patients referred to an outpatient service for suspected treatment-resistant schizophrenia (TRS) has revealed that a substantial proportion of these individuals, once carefully reassessed by specialists and reclassified according to ICD-10 criteria, turned out to have conditions falling well outside the schizophrenia category (F20). Instead, many were reclassified as having organic psychotic disorders (F06) or unspecified psychosis (F29), categories with profoundly different underlying causes, prognoses, and treatment implications.

The study, conducted by João Gama-Marques and colleagues at Lisbon’s specialised resistant schizophrenia outpatient service and published in the journal Schizophrenia, offers an unusually clear window into a problem that clinicians have long suspected but rarely quantified so directly. When patients who have failed multiple antipsychotic trials arrive at a tertiary referral centre, the assumption is often that they represent a relatively uniform population: people whose schizophrenia has proven stubbornly resistant to first- and second-line medication. The new findings challenge that assumption at its foundation, showing that the very label of suspected treatment resistance may bundle together diagnostically distinct groups of patients whose illnesses arise from fundamentally different mechanisms.

Understanding why this matters requires appreciating what treatment-resistant schizophrenia actually is. Roughly one in three people with schizophrenia does not respond adequately to standard antipsychotic medications, even when these drugs are taken at appropriate doses for sufficient durations. For this population, clozapine stands alone as the only medication with robust evidence of efficacy where other antipsychotics have failed. Yet clozapine is dramatically underused worldwide, constrained by its requirement for regular blood monitoring due to the risk of agranulocytosis, a potentially dangerous depletion of white blood cells, along with a burdensome side-effect profile that includes metabolic changes, sedation, constipation, and seizures. Guidelines from multiple countries therefore reserve clozapine for patients who meet strict criteria for genuine treatment resistance, making accurate diagnosis a gatekeeping step in whether the drug is ever offered.

This is precisely where diagnostic heterogeneity becomes clinically consequential. In the Portuguese cohort, the researchers reviewed electronic clinical records and reclassified every referred patient according to ICD-10 diagnostic codes after specialist reassessment. Patients with confirmed schizophrenia (F20) and schizoaffective disorder (F25) emerged as the groups with the highest clozapine exposure and the greatest overall antipsychotic burden, a pattern consistent with the expectation that clinicians intensify pharmacological treatment when a primary psychotic disorder genuinely resists standard regimens. By contrast, patients classified in the organic psychotic disorder (F06) and unspecified psychosis (F29) categories showed markedly lower pharmacological intensification, suggesting that once reassessment revealed a different diagnostic picture, treating clinicians were more cautious about escalating medication.

The statistical analysis sharpened this picture further. When the investigators examined factors associated with clozapine exposure across the diagnostic groups, an unspecified psychosis diagnosis (F29) was independently associated with lower odds of clozapine exposure compared with schizophrenia (F20). In other words, even after accounting for other variables, the diagnostic label itself predicted whether patients received the drug of choice for genuine treatment resistance. This relationship carries a double significance. On one hand, it may reflect appropriate clinical restraint: clozapine’s evidence base rests on trials conducted in schizophrenia, and extending it to psychoses of uncertain or organic origin carries uncertain benefit and real risk. On the other hand, it raises the possibility that some patients within these heterogeneous groups might be missing out on effective treatment because diagnostic uncertainty, rather than treatment resistance itself, is shaping therapeutic decisions.

Organic psychotic disorders deserve particular scrutiny in this context. The F06 category encompasses psychotic symptoms arising directly from brain injury, neurological disease, or other medical conditions affecting the brain. Psychosis in these settings can mimic schizophrenia closely, featuring hallucinations and delusions that look superficially identical, yet the underlying pathology, and the appropriate treatment target, may be entirely different. Antiepileptic management, treatment of an underlying infection or autoimmune process, or adjustments to a causative medication may resolve symptoms that no antipsychotic could touch. A patient with such a condition who has failed two or more antipsychotic trials is not treatment-resistant in the schizophrenia sense; they have been treated for the wrong illness. The Lisbon findings suggest that this scenario occurs often enough among TRS referrals to reshape how such referrals should be evaluated.

The unspecified psychosis category (F29) presents a related but distinct challenge. It functions as a residual diagnosis, applied when psychotic symptoms are evident but the information available is insufficient to determine whether the illness is schizophrenia, an affective psychosis, an organic condition, or something else. That such a category featured prominently among referrals for suspected treatment resistance tells its own story: patients who have bounced through multiple treatment failures often accumulate fragmented clinical histories, and the diagnostic signal may become progressively noisier as episodes, medications, and institutional transitions pile up. Reassessment by specialists with the time and expertise to reconstruct that history appears to be a powerful tool for restoring diagnostic clarity, and the study’s authors argue that this reassessment step is genuinely important rather than merely administrative.

The study’s most consequential conclusion may be its caution about how clozapine use itself is interpreted. In health-services research, clozapine prescription is frequently used as a proxy marker: if a patient is on clozapine, researchers often infer that treatment resistance was recognised and appropriately treated. The new findings show why this inference is unsafe. Because clozapine exposure tracks the diagnostic category so closely, and because diagnostic categories themselves vary so widely among TRS referrals, clozapine use cannot be read as a stand-alone indicator of treatment-resistant schizophrenia status. Two patients on clozapine may sit in entirely different diagnostic and therapeutic contexts, and a registry that counts them identically will misdescribe the very phenomenon it aims to measure. The authors explicitly recommend that clozapine exposure be interpreted within the diagnostic and treatment context rather than in isolation.

These results also speak to a broader and increasingly urgent conversation about diagnostic precision in psychiatry. Schizophrenia, as defined by current classification systems, is almost certainly a heterogeneous syndrome rather than a single disease, and the boundaries separating it from schizoaffective disorder, bipolar psychosis, organic psychoses, and undefined psychotic states remain contested. Studies of referred populations, by concentrating the most complex and treatment-refractory cases, act as stress tests for these boundaries. When nearly a diagnostic category’s worth of referred patients shift classification under specialist scrutiny, the finding suggests that referral labels travel poorly: what a community clinic records as resistant schizophrenia may be something quite different by the time it reaches a specialised service with resources for thorough reassessment. For epidemiologists, this means that prevalence estimates for treatment-resistant schizophrenia derived from administrative data may be systematically distorted. For trial designers, it means that enrolling patients based on referral status alone risks diluting study samples with diagnostically mixed populations, potentially obscuring treatment effects.

For clinicians, the practical message is a call for humility and thoroughness at the point of referral evaluation. Before concluding that a patient has treatment-resistant schizophrenia, the assessment should verify the diagnosis itself, not merely the adequacy of prior medication trials. Checking adherence, ruling out organic causes, reviewing substance use, and reconstructing the full treatment history are all standard recommendations, but the Lisbon data demonstrate empirically that diagnostic reclassification changes outcomes in a substantial share of real referrals. The work, supported by Portuguese national funding through the Foundation for Science and Technology, ultimately reframes suspected treatment resistance not as a single clinical problem but as a gateway question that demands diagnostic precision first. Only when the underlying condition is correctly identified can decisions about clozapine, the most powerful and most demanding tool in the antipsychotic arsenal, be made on solid ground, ensuring that the patients who genuinely need this last-line medication receive it, and that those whose psychoses stem from other causes are redirected toward treatments that might actually help them.

Subject of Research: Diagnostic reclassification and clozapine use among patients referred for suspected treatment-resistant schizophrenia

Article Title: Diagnostic heterogeneity and clozapine use among patients referred for suspected treatment-resistant schizophrenia

Article References: Gama-Marques, J., Schumacher, M. M., & Henriques-Calado, J. (2026). Diagnostic heterogeneity and clozapine use among patients referred for suspected treatment-resistant schizophrenia. Schizophrenia. https://doi.org/10.1038/s41537-026-00812-4

Image Credits: AI Generated

DOI: 10.1038/s41537-026-00812-4

Keywords: treatment-resistant schizophrenia, clozapine, diagnostic heterogeneity, ICD-10, organic psychotic disorders, unspecified psychosis, schizoaffective disorder, antipsychotic treatment, psychiatric diagnosis, schizophrenia research, referral clinics, psychosis