Inclusion Criteria:
* Unexplained cytopenia for at least 6 months. Cytopenia is defined as the presence of ≥1 blood count indexes below the following thresholds:
* Hgb \<10 g/dL
* ANC \<1.8 × 10\^9/L
* Platelets \<100 × 10\^9/L
* IDH1 gene mutation (R132) confirmed by droplet digital PCR (ddPCR) testing, at a frequency \> 2%. This will be performed locally and confirmed at Washington University.
* At least 18 years of age.
* ECOG performance status 0-2
* Adequate organ function as defined below:
* AST(SGOT)/ALT(SGPT) ≤ 3.0 x IULN
* Serum total bilirubin \< 1.5 x IULN (an upper limit of bilirubin 5mg/dL is acceptable if it can be attributed to Gilbert's syndrome or erythropoiesis)
* Serum creatinine \< 2 x IULN or creatinine clearance \> 50 mL/min by Cockcroft-Gault glomerular filtration rate estimation
* The effects of ivosidenib on the developing human fetus are unknown. For this reason, women of childbearing potential and men must agree to use adequate contraception (defined in Section 5.5) prior to study entry, for the duration of study participation, and for 90 days after the last dose of ivosidenib. Should a woman become pregnant or suspect she is pregnant while participating in this study, she must inform her treating physician immediately. Men treated or enrolled on this protocol must also agree to use adequate contraception prior to the study, for the duration of the study, and for 90 days after the last dose of ivosidenib.
* Ability to understand and willingness to sign an IRB approved written informed consent document (or that of legally authorized representative, if applicable).
Exclusion Criteria:
* Indication of hematologic disease by bone marrow biopsy within 6 months of study entry.
\*Evidence of disease progression from time of bone marrow biopsy to enrollment based on investigator review of symptoms and complete blood counts
* Active malignancy (defined as \> 1 cm disease on most recent CT scan in the past 6 months).
* Currently receiving therapy for solid tumor malignancy.
* Currently receiving any other investigational agents.
* Known dysphagia, short-gut syndrome, gastroparesis, or other conditions that limit the ingestion or gastrointestinal absorption of drugs administered orally.
* A history of allergic reactions attributed to compounds of similar chemical or biologic composition to ivosidenib or other agents used in the study.
* Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, or cardiac arrhythmia.
* Pregnant and/or breastfeeding. Women of childbearing potential must have a negative pregnancy test within 72 hours of study entry.
* Heartrate corrected QT interval (QTc) \> 450 msec or with other factors that increase the risk of QT prolongation or arrhythmic events (e.g. heart failure, hypokalemia, family history of long QT interval syndrome).
* Known medical history of progressive multifocal leukoencephalopathy (PML).
* Currently taking medications known to be CYP3A4 strong inducers and sensitive substrates.
