Would you target the same hydroxychloroquine blood level for discoid lupus compared to systemic lupus?

Yes, I would. 

A French multicenter study (under the direction of Nathalie Costedoat-Chalumeau, MD, one of my favorite HCQ experts) assessed whole blood HCQ drug levels in 300 patients with cutaneous lupus erythematosus (CLE). There were 254 cases of chronic cutaneous lupus, 160 of which were discoid lupus erythematosus (DLE). There were also 86 cases of subacute cutaneous lupus (4).

Those in remission had an average HCQ drug level of 910 ng/mL, those with partial remission had an average of 692 ng/mL, and nonresponders had an average of 569 ng/mL. There was clearly an association between drug level and disease control. 

Some caveats to HCQ drug level testing:

1. We should all be checking whole blood HCQ drug levels regularly as the standard of medical care in our lupus patients. Both Labcorp and Quest now offer HCQ whole blood levels. We should all be ordering them regularly just like we do anti-dsDNA, C3, C4, and urine protein creatinine ratios! (10). 
2. Levels are best drawn right before a dose (trough levels) because blood levels can rise as much as 30% after a dose, peaking around 4 hours after consumption (2). An interesting thing with this is that low levels certainly are indicative of poor adherence (no matter the timing). However, that same poorly adherent patient could have a "therapeutic drug level" if they just happen to get their blood work 4 hours after their dose.
3. My whole blood HCQ drug level target is 1000 ng/mL to 1200 ng/mL. Petri, et al showed that systemic lupus erythematosus (SLE) patients with levels less than 1068 ng/mL had a greater risk for thrombotic events (8).
   
   It may not be commonly known that CLE increases the risk for cardiovascular events (5). So, preventing thrombotic events is equally important in our DLE patients.
   
   Also, levels less than 1000 ng/mL are associated with a significantly greater risk for SLE flares (3). 
   
   When the HCQ drug level gets above 1200 ng/mL, patients begin to have a greater risk for the most concerning side effect, antimalarial retinopathy, per the Johns Hopkins Lupus Cohort (7). The facts in these last several paragraphs are my rationale for 1000 to 1200.
   However, in reality, it is hard to keep levels in this narrow range. So, I realistically aim for 1000 to 1500 ng/mL.
   
   In the retinopathy cohort mentioned above, retinopathy risk was by far the greatest in the top tertile drug level group with levels greater than 1752 ng/mL. If an SLE patient requires a level 1500 - 2000 to stay in remission, then I'll accept a slightly higher level if that can prevent them from requiring an immunosuppressant (plus, I ensure yearly SD-OCT and VF 10-2 in all HCQ patients (1). 
   
   Do not forget to get a 3rd test yearly in your Asian patients (a VF 24-2 or VF 30-2; [9]).
   
   
4. Ensure you order whole blood HCQ drug levels. I know that LabCorp also can do serum HCQ drug levels, and they are not useful. When our office first started ordering levels from LabCorp, they accidentally performed serum levels at first, and they were worthless.
5. Aim for remission in DLE; it is a major driver of poor quality of life in our patients (6).

REFERENCES:

1. Aduriz-Lorenzo et al., PMID 32316844
2. Al-Rawi et al., PMID 28862574
3. Costedoat-Chalumeau et al., PMID 17009263
4. Francès et al., PMID 22508872
5. Hesselvig et al., PMID 27235406
6. Ogunsanya et al., PMID 30637718
7. Petri et al., 2018 ACR/ARHP Annual Meeting, ABSTRACT NUMBER 2897
8. Petri et al., PMID 33403833
9. Stokkermans et al., NCBI NBK537086 
10. Thomas et al., The Rheumatologist FEB 2020. Retrieved on 1/4/21