By Dr. Jean-Marc Lucas, DO, MA. Founder of Premeducated, former medical school professor, full-time COMLEX and USMLE tutor, 99th percentile on COMLEX. Book a free 1:1 strategy call if you want a personalized plan.
Do both, in order. Start with system-based (focused) blocks on the weak areas your score report identifies, then switch to random (mixed) blocks and stay there through test day. Focused blocks repair a defined weakness. Mixed blocks test whether you can retrieve the answer when nobody tells you which system the question belongs to, which is exactly how COMLEX asks it.
The mistake is treating this as a personality choice. Students who only do system-based blocks get inflated percentages and a rude surprise on their next COMSAE. Students who only do random blocks spread their reps evenly across topics they already know and topics that are sinking their score. The order fixes both problems.
What is the difference between random and system-based blocks?
A system-based block pulls every question from one system or discipline, or from a small group of related ones. UWorld, TrueLearn, and COMQUEST all let you filter this way. You'll also see these called focused blocks, which is the term I use with my students.
A random block pulls from the entire bank with no filter. One question is a pediatric rash, the next is a rib dysfunction, the next is biostatistics. These are also called mixed blocks.
The two block types train different skills. Focused blocks build knowledge in one area through concentrated repetition. Mixed blocks train recognition, the step where you read a vignette cold and decide what kind of problem you're looking at. COMLEX tests both, but it only ever shows you the second format.
Why do system-based blocks inflate your score?
When you open a cardiology block, the diagnosis is half announced before you read the first word. A patient with chest pain in a cardio block has an MI, a dissection, or pericarditis. You're picking between three or four cardiac answers.
Put the same vignette in a mixed block and the differential gets much wider. Chest pain could now be a pulmonary embolism, an esophageal rupture after vomiting, a panic attack, costochondritis, or a rib somatic dysfunction. On COMLEX, it could also be a question about the viscerosomatic reflex at T1 to T5 where the chest pain is just the setup. The block title did a chunk of your thinking for you, and test day doesn't come with block titles.
That's why a student can score well on system blocks and then watch the number drop when they switch to random. The knowledge didn't disappear. The hint did.
There's a second, quieter problem. In a single-system block, you start picking the cardio answer because you know you're in a cardio block, without working through the stem. That habit follows you into the real exam, where a distractor from the wrong system is sitting in the answer choices waiting for you.
When should you use system-based blocks?
Use them when you have a defined weakness to repair. That means a system or discipline that your COMSAE or COMLEX score report flags as low, weighted by how much of the exam it represents. If you haven't done that ranking yet, start with how to interpret your score report.
Here are the block counts I give my 1:1 students:
- Weak disciplines: 4 to 6 full-length blocks each
- Weak systems: each one gets 3 to 6 full-length blocks
Those are totals across all of dedicated, not daily numbers. Start at the low end of the range. Add more only if you're scoring well below average on those specific blocks.
Concentrated reps work for repair because you see the same concepts from several angles in a short window. Miss a nephritic syndrome question on Monday, review it properly, and you'll probably get a different nephritic question on Tuesday. That second exposure, while the first one is still fresh, is what makes the fix stick. Random blocks can't give you that. In a mixed block, your next nephritic question might be several blocks away.
Focused blocks also pair well with Anki. The cards you make from a renal block get reinforced by the next renal block. Cards tied to information you recently learned or relearned are the most efficient cards you'll do.
How do you keep system-based blocks from giving away the answer?
Combine similar weak systems into one block. If your circulatory/hematology and respiratory scores are both low, don't do separate cardio blocks and pulm blocks. Do combined circ/heme/resp blocks.
Now a patient with shortness of breath could have heart failure, a pulmonary embolism, COPD, or anemia, and you have to work the stem to tell them apart. You still get concentrated reps on your weak areas. You lose the freebie.
Here's an example focused-block plan I built from a real student score report:
- OPP: 4 to 6 blocks in TrueLearn or COMQUEST
- MSK/Neuro: about 23% of the exam, so 4 to 6 blocks
- Circ/Heme/Resp: roughly 20% of the exam, another 4 to 6 blocks
- GI/Repro: 3 to 5 blocks for about 15% of the exam
Notice there's only one discipline on that list. OPP is a large share of the test and it's specific enough to study directly. Internal medicine is also a huge discipline, but it spans all of medicine, so the combined system blocks are how we target it. I go toward the higher end of the block range for combination blocks, because each one is covering more ground.
The full method for building this list is in how to pick focused question blocks based on weak areas.
When should you switch to random blocks?
Switch when you run out of focused blocks on your list. That's the whole rule. There's no percentage you have to hit first.
In the schedule I give most of my 1:1 students, the two timed blocks each morning are focused blocks until the list is finished, then mixed blocks from that point on. Once you've finished your final focused blocks, you do mixed blocks through test day. For most students, that puts the mixed phase across roughly the final two weeks of dedicated, alongside the last practice tests.
Expect your percentage to dip in the first few mixed blocks. That dip is the block-title hint going away, and it's normal. What you're watching for is the trend over the next week or two.
If the mixed blocks expose a system that's still clearly weak, you can go back and add a couple of focused blocks for it. The ranges exist for that reason. Don't use it as an excuse to retreat to system blocks because the random scores feel worse. A mixed-block score that feels worse is also closer to the truth.
If you'd like a second set of eyes on your block list, the free Premeducated Skool community is where students post their plans and get feedback.
What do random blocks tell you that system blocks can't?
They tell you whether you're ready. A mixed block is the closest thing your question bank has to a COMLEX block: 40 questions, one hour, every system and discipline in play, no warning about what's coming.
That makes your mixed-block performance a usable readiness signal, and your system-block performance a poor one. When I'm helping a student decide whether to postpone, one of the things I look at is recent question bank performance. TrueLearn shows your percentile over your last 100 questions. I'll sometimes have a student do 100 mixed questions, record the percentile, repeat that two or three times, and average the results. That number only means something if the questions were mixed.
Mixed blocks also catch the topics your focused list never covered. You picked focused blocks based on your weakest areas, so your middle-of-the-pack systems got no dedicated reps. Random blocks keep those warm through test day so they don't decay while you're busy fixing renal.
Every block, focused or mixed, should be timed and out of tutor mode. The reasoning is in timed vs tutor mode for COMLEX question banks.
Which question bank should you use for each type?
For focused blocks, I point students to UWorld for most systems. The exception is OMM and MSK, where I use COMQUEST. For OPP specifically, TrueLearn works too.
For the mixed phase at the end of dedicated, I suggest TrueLearn or COMQUEST. They're written for COMLEX, so the question style and the OMM sprinkled through the blocks look more like test day.
If you bought the NBOME's WelCOM question packs, use them as mixed blocks, the same way you'd use a random TrueLearn or UWorld block. Don't try to sort them by system.
If you only have one bank, the sequence still holds. Filter by system for the focused phase, then turn the filter off. The full bank comparison is in UWorld vs TrueLearn for COMLEX.
What about biostatistics, ethics, and OMM at the end?
These are the one place where I add focused blocks back in during the final two weeks. They're memorization-heavy, they show up reliably, and students postpone them for months.
In the last stretch, plan 1 to 2 biostatistics blocks and 1 to 2 ethics and jurisprudence blocks on top of your mixed blocks. For biostats, make sure you're comfortable with sensitivity and specificity, relative risk, absolute risk reduction, NNT and NNH, odds ratios, type I and type II errors, study designs, and biases. For ethics, know emancipated minors, EMTALA, HIPAA, Stark law, and anti-kickback. The rest of that window is covered in what to do in the final two weeks before COMLEX.
These are short, targeted patches. They don't replace the mixed blocks, which stay as your daily default until the exam.
Frequently asked questions about random vs system-based blocks
Should I do random or system-based blocks in UWorld?
Both, in sequence. Use UWorld's system filter first for the weak areas on your score report: 4 to 6 blocks per weak discipline and 3 to 6 per weak system, combining related systems into a single block where you can. When that list is finished, turn the filter off and do random blocks through test day. For OMM and MSK focused blocks, use COMQUEST or TrueLearn instead.
Why did my score drop when I switched to random blocks?
In a system-based block, the block title narrows the differential before you read the stem. Chest pain in a cardio block is a cardiac diagnosis. In a random block that same chest pain could be pulmonary, GI, psychiatric, or musculoskeletal, so you have to do the recognition step yourself. A dip in the first few mixed blocks is normal and doesn't mean you lost knowledge. Watch the trend over the following one to two weeks instead of the first few scores.
Should I do random blocks during my preclinical years?
During systems courses, it makes sense to filter by the system you're currently studying, because a random block would be full of material you haven't been taught yet. That's a different situation from dedicated. Once you've covered the full curriculum and you're in dedicated, the focused-then-mixed sequence applies: repair weak areas with focused blocks first, then go fully mixed.
How many focused blocks should I do before going mixed?
Plan 4 to 6 full-length blocks for each weak discipline and 3 to 6 for each weak system. Those are totals across dedicated, not per day. Start at the low end and add more only if you're scoring well below average on those specific blocks. When the list is done, switch to mixed blocks. There's no target percentage you have to reach before you're allowed to switch.
Can I skip system-based blocks and only do random?
You can, but it's a slow way to fix a weak area. If renal is dragging your score down, a random block might give you only a handful of renal questions. A focused plan gives you several full blocks of renal and related systems in a short window, so you see the same concepts repeatedly while your review is still fresh. Students with no clear weak areas on their score report can reasonably go mixed earlier.
Should focused and mixed blocks both be timed?
Yes. Every block should be timed and out of tutor mode, regardless of type. COMLEX gives you 40 questions in a one-hour block with no feedback until the exam ends. Timing is a learned skill, and students who live in tutor mode often struggle to finish on test day. Do the learning afterward, in your question review.
Get feedback on your block plan in the free Skool community
If you're not sure which systems belong on your focused list, or whether it's time to go mixed, bring your score report to the free Premeducated Skool community. It has weekly office hours, question breakdowns, and DO students working through the same decisions.